And I can make sure I keep out of it. "Look, I'd better get out to the platform," he said.

"Wait," Eileen said, grabbing his sleeve. "What happened?"

"What happened?" he repeated blankly.

"To Ross's son?" Polly asked.

"No, to Shackleton. When he left his crew on the island and went off to get help. Did he come back?"

"Yes, with a ship, to take them all home. He didn't lose a single man."

"Good," she said, and smiled at him.

"Ring us as soon as you get there," Polly said.

"I will," he promised, thinking, If I can get there. Just because he'd gone to one divergence point didn't mean the continuum would let him near another, especially one where a single person could mess up everything. His train could be blown up en route. Or the train might be too crowded to get on, which looked like it was going to be the case.

It was packed to the gills, but he managed to squeeze on, and on the train from Oxford, he was even able to find a seat-taking care to pick a compartment that didn't have any blonde stammerers, tall pipe-smokers, or doped-up children in it. He picked one occupied by five soldiers and two elderly ladies. He slung his bag up onto the luggage rack-which only held brown-paper-wrapped packages, no children-and sat down in the single empty seat.

He was almost instantly sorry. As soon as the train pulled out of the station, the soldiers left the compartment to go have a smoke, and a bald, spectacled man dressed in tweeds, with a knitted vest even rattier and more full of holes than the one Eileen had found for Mike came in and sat down between Mike and the door, stretching out his legs so it was impossible for Mike to get out of the compartment without asking him to move, and he didn't want to have any contact with him.

The man was too bald to be Turing and too short to be Knox, and he didn't have a red beard, but he definitely worked at the Park. The moment the train left the station, he pulled out a book titled Principia Mathematica and buried his nose in it, ignoring Mike and the two ladies, who were cheerfully discussing various physical ailments.

"The pain begins in my foot and works its way all up my spine," the one in the brown hat said. "Dr. Granholme says it's sciatica."

"I have a dull throbbing pain in my knees," the other one, in a black hat with a bird on it, said. "Dr. Evers prescribed a course of nutrient baths, but it didn't do a bit of good."

"You should go to Dr. Sheppard in Leighton Buzzard. My friend Olive Bates says he's wonderful with knees. I didn't tell you, her son was called up last week. Poor Olive, she's frightfully worried he'll be sent somewhere dangerous."

Like Bletchley Park, Mike thought, pretending to look out the window. BP was an exponentially more dangerous divergence point than Dunkirk because it involved a secret, and secrets were the most fragile and easily altered divergence points in the continuum. Because even though it took the combined efforts of many people to keep a secret, a single person, a single careless remark, could reveal it. Like a delayed-action bomb, which the slightest touch could set off.

All he had to do was ask the wrong question. Or too many questions. Or blow his cover. That meant he'd have to watch every word. His American L-and-A still hadn't worn off, so he'd have to remember to keep his vowels clipped and to use the English terms for things. No "flashlights" or "elevators," though he doubted Bletchley was a big-enough town to have elevators-correction, lifts-and it- The train jerked to a stop. Black Hat with Bird looked nervously out the window. "Oh, dear, I do hope it's not an air raid. I'd hoped to arrive in Bletchley before dark."

And I'd hoped to arrive in Bletchley, period, Mike thought, hoping a passing troop train had delayed them, but they weren't on a siding, and after a minute the guard came through apologizing for the delay and asking them to pull down the blackout blinds.

"Is it a raid?" Brown Hat asked.

"Yes, madam," the conductor said, "but I'm certain there's no danger."

Except from me, Mike thought, listening for approaching planes, but nothing happened. They didn't start up again either, and as they sat there, everything Polly'd told him about how she'd influenced the shopgirl Marjorie came back to him, and he found himself thinking about Dunkirk and all the other things he'd done besides unfouling that propeller, from tossing those gas cans overboard to hauling the dog up over the side. He'd lost his life jacket in the water. Had it floated off somewhere to entangle itself in some other propeller? And what about the body? And now here he was going to a place where a single mistake, a single word, could- The train jerked sharply and started moving again, and the ladies went back to discussing their ailments. "All autumn I've had a dreadful pain in my heel," Brown Hat said. "A friend of mine told me about Dr. Pritchard's manipulation treatments, so I'm going to his clinic in Newport Pagnell."

"Newport Pagnell?" Black Hat with Bird cried. "Why, that's quite near Bletchley! You must come for tea one day. Are you getting off there, too?"

"Yes. Dr. Pritchard's sending a car."

Good, that meant he wouldn't have to ask the spectacled man which station was Bletchley.

"If Dr. Pritchard's treatment isn't satisfactory," Black Hat with Bird went on, "you must go to Dr. Childers in St. John's Wood."

St. John's Wood. The lab had had a permanent drop there in the early days of time travel, before they'd figured out how to set up remotes. He wondered if Polly or Eileen knew where it was. When their drops malfunctioned, the lab might have reopened it to use as an alternative. He would have to tell Eileen and Polly that when he called them-correction, rang them up-to tell them he'd arrived safely.

If they ever got there. He had to sit through a seemingly endless discussion of bunions, rheumatism, lumbago, and palpitations before Black Hat with Bird said, "Oh, good, we're coming into Bletchley," and both ladies began collecting their things. The man continued reading even when they pulled in to the station, and Mike wondered if he'd been wrong about him being one of Bletchley Park's cryptanalysts. But the second the train stopped, the man clapped his book shut and, without so much as a glance at any of them, was out the door and walking rapidly along the platform toward the station. Mike stood up, intending to follow him, but the ladies asked him to help them take their packages down from the overhead rack, and by the time Mike did, the man had vanished.

But there were plenty of people still in the station and outside-unlocking bicycles and walking away from the station-whom he could follow. As soon as he found a phone. He'd promised Polly he'd call to tell her he'd got there okay. He only hoped it didn't take forever to put the call through.

The phone booth-correction, box-wasn't occupied, and the operator put the call through fairly quickly, but Mrs. Rickett answered and, when he asked for Polly, said sourly, "I don't know if she's here," and when he asked her to go check, gave a put-upon sigh and went off for so long he had to put more coins in.

When Polly finally answered, he said, "I've got to make this quick." The stuff about St. John's Wood could wait till next time. "I got here all right."

"Have you found a room? Or Gerald?"

"Not yet for either one. I just got off the train. I'll call you as soon as I know where I'm staying," he said, then hung up and hurried out into the station, but it had already emptied out, and when he went outside into the gathering dusk, there was no one in sight.

I should've watched to see which way they were all going and then called, he thought, kicking himself. Well, it was too late now. It was already getting dark. He'd have to wait till tomorrow morning to find out where Bletchley Park was. Right now he needed to find the center of town and a room. But there wasn't a taxi in sight either, and no sign saying To City Centre.

He set off along the likeliest-looking street, but its brick buildings quickly gave way to warehouses, and when he reached the corner, he couldn't see anything promising in either direction. This is ridiculous, he thought. How big can Bletchley be? If he kept walking, he'd eventually have to come to something, even if it was only the edge of town, but it would be completely dark in a few more minutes, and his bad foot was beginning to ache. He looked up the side street again, trying to decide which way to go.

And glimpsed two people in the dusk. They were a block and a half away-too far ahead for him to catch up to them with his limp, but he hobbled after them anyway.

The pair reached the corner and stopped, as if waiting to cross, even though there weren't any cars he could see. Mike labored to catch up to them. It was two young women, he saw as he got closer, obviously two of the hundreds Polly had said worked at Bletchley Park. Good. After he'd asked them for directions, he could say, "You wouldn't happen to know a Gerald Phipps, would you?" and since Phipps was such a jerk, they'd make a face and say, "Why, yes, unfortunately we do," and he could be on the train back to London to pick up Eileen and Polly by tomorrow.

Only half a block to go. The young women were still standing there talking, totally caught up in what they were saying and oblivious to his approach. And no wonder they'd been known as "girls." They didn't look more than sixteen. They were talking animatedly and giggling, and it was clear as he got closer to them that they weren't waiting to cross. They'd simply stopped to talk.

Keep talking till I catch up, girls, he willed them, but when he was still a hundred feet away, they crossed the street, walked to the second building, and started up the steps to the door.

Oh, no, they were going in. He hobbled quickly to the corner. "Hey!" he called, and both girls turned at the door and looked back at him. "Wait!" He stepped out into the street. "Can you tell me the way to-"

He didn't even see the bicycle. His first thought as his bag flew out of his hand and both palms and his knee hit the pavement was that a bomb must have exploded and the blast had knocked him down, and he looked toward the girls, afraid they'd been hit, too. But they were running down the steps and over to him, exclaiming, "Are you all right? Did he injure you?"

"He?" he said blankly.

"When he ran his bicycle into you," the first girl said, and it was only then that he realized he'd been knocked down by a bicyclist. He looked on down the street to see the bicycle wobble and swerve and then crash into the curb, spilling its rider onto the pavement and clattering onto its side.

The girls saw the bike crash, too, but they paid no attention, even though it looked like the rider had taken a much worse fall than Mike had. They were busy picking Mike up. "Are you injured?" the first girl asked anxiously, putting her hand under Mike's arm to help him up.

"I think he only clipped me," Mike said.

The other was standing with her hands on her hips, staring at the rider, who was getting slowly to his feet. "He shouldn't be allowed on the streets," she said, annoyed.

"Do give us a hand, Mavis," the first one said to her, and Mavis came over to take Mike's other arm. Mike stood up, more or less. "Are you certain you're not hurt?" she asked.

"I don't think so," he said, taking stock. His knee was beginning to throb, but he was able to put weight on it, so it wasn't broken or sprained, and it and his hands had struck the pavement first. He flexed his fingers. "I think I'm fine. Anyway, nothing's broken. I should have been looking where I was going."

"You should have?" Mavis exploded. "He should have. It's the third time he's knocked someone down this week! Isn't it, Elspeth?"

Elspeth nodded. "He nearly killed poor Jane on her way to the Park last week." She glared at the rider, who was righting his bicycle. He got on and rode off down the street, apparently unharmed. "Watch where you're going!" she shouted after him, to no effect. He didn't even look back.

"You're certain you're all right?" Mavis was asking. "Oh, you're limping."

"No, that isn't from-"

"I knew he'd end by injuring someone," Mavis said angrily. "He never watches where he's going."

"My foot's not hurt," Mike said, but neither girl was listening.

"He's an absolute menace," Mavis said angrily. "He should be forbidden from riding a bicycle."

Elspeth shook her head. "He'd only begin driving his car again, and that would be even worse," she said. "Turing's a wretched driver."

In wartime the truth is so important it must be accompanied by a bodyguard of lies.

WINSTON CHURCHILL, IN A.

SPEECH AT BLETCHLEY PARK.

London-November 1940 POLLY AND EILEEN WAITED TO MAKE SURE MIKE'S TRAIN actually left for Bletchley Park, and then Eileen went to Whitechapel to return Alf Hodbin's map. "I told them I'd post it to them, but I promised Theodore Willett I'd go see him, so I may as well run it by. And I want to talk to Alf. I got the feeling last time that he and Binnie are up to something."

"Like what?" Polly asked.

"I'm not certain, but knowing the Hodbins, it's something illegal. There weren't any Nazi child spies, were there?"

Polly saw her to her train and then went to the British Museum-"Darling, so sorry. If you can forgive me, meet me by the Rosetta stone Sunday at two"-to wait for the retrieval team. And fret.

In spite of Mike's reassurances that they hadn't affected events, she was still worried. Her actions hadn't affected only Marjorie. They'd also affected the warden who'd found her and the rescue squad and ambulance driver, her nurses and doctors, the airman she hadn't met who'd gone off on his mission thinking she'd changed her mind about eloping, even Sarah Steinberg, who'd been given Marjorie's job, and the shopgirl Townsend Brothers had hired to replace Sarah. The ripples spread out and out. And now Marjorie was going to be a nurse. She was going to be saving soldiers' lives.

Like Mike had saved Hardy's. And unlike Hardy, there was nothing else which could have caused what had happened. Marjorie had said quite plainly that she'd decided to run off with her airman because of having seen Polly standing there looking so shell-shocked the morning after St. George's was hit. That had led directly to her having been in Jermyn Street when it was hit, and to her deciding to become a nurse and thus altering who knew what other events. Polly saw now why Mike had been so worried that morning outside Padgett's when he thought he'd saved Hardy.

And now Mike was on his way to Bletchley Park, where he could do far more damage to the war than a hospital nurse could. If Gerald Phipps hadn't already beaten him to it.

But if he had, there should be more discrepancies than just a siren going off when it shouldn't have. And Mike was right, there were all sorts of instances in history when an action which should have had a major effect had been counteracted by something else, like the Verlaine-poem invasion signal. Or the appearance of "Omaha" and "Overlord" in the Herald's crossword puzzle, which hadn't affected the invasion after all.

But that was also an example of how a single small action could have tremendous consequences. A few words in a crossword puzzle had nearly derailed an invasion involving years of careful planning and two million men. If D-Day had had to be delayed, the invasion's location would almost certainly have leaked out, and Rommel's tanks would have been waiting for the invasion troops at Normandy. And all because of a bit of carelessness and a teenaged boy. "For want of a nail ..."

So what sort of impact could the combined actions of Marjorie and Hardy, and Gerald's and now Mike's wandering around the place where the most important secret of the war was being kept have? If Mike got there. Just because he'd gone to Dunkirk didn't mean he'd be allowed to reach Bletchley Park.

She gave the retrieval team another half hour to show and then went back to Mrs. Rickett's to see if Mike had phoned. He hadn't, and by the time Eileen returned, there'd still been no word from him. "Did you find out what the Hodbins were up to?" Polly asked her.

"No, no one was there," Eileen said, frowning. "I had to slide the map under the door. Did Mike ring up?"

"No, not yet. His train was likely delayed by a troop train or something."

But she must not have succeeded in hiding her anxiety, because Eileen asked, "No trains were bombed today, were they?"

"No." Not in London.

"Was Bletchley bombed?"

"I don't know, but there were never any casualties at Bletchley Park. Come along, it's time for supper. One of Mrs. Rickett's Sunday-night 'cold collations.' "

Tonight it consisted of sliced tongue and nettle salad. "I'm sorry I ever got my ration card," Eileen said when she saw it. "I can't wait till Mike finds Gerald and we can go home. Perhaps that's why he hasn't rung us, because someone on the train knew where Gerald was, and he's gone off to find him."

But when Mike finally rang up, moments before Polly had to leave for Notting Hill Gate for rehearsal, it was only to say he'd arrived. He hadn't even left the station yet. And he was in a hurry. He told them he'd phone them again when he knew where he was staying and rang off before Polly could warn him to be careful.

But if the problem's an increase in slippage, then it would have prevented him from going to Bletchley Park if he could affect events. There's nothing to worry about, she told herself, and forced herself to concentrate on the problems of the admirable Crichton and Lady Mary.

The troupe was in their final week of rehearsals, and Sir Godfrey was in a foul mood. "No, no, no!" he shouted at Viv. "You say, 'Here comes Ernest,' before Ernest makes his entrance! Again. From 'Father, we thought we should never see you again.' "

They started through the scene again.

"No, no, no!" Sir Godfrey thundered at Mr. Dorming. "Why can't you remember? This is a comedy, not a tragedy. At the end of Act Three you are rescued from this island."

"By a prince?" Mrs. Brightford's little girl Trot asked.

"No, by a ship. Or, considering the rate at which this production is progressing, by the end of the war."

"I think it should be by a prince," Trot said.

"Take it up with the author," Sir Godfrey growled. "Try it again. From 'Here comes Ern-' "

"Sir Godfrey," Lila interrupted. "You keep saying it's a comedy, but how can it be when Lady Mary and Crichton are separated at the end?"

"Yes," Viv said, "and why can't they be together?"

"Because he is a butler and she is a lady. You and Mary," he said, glaring at Polly as though this was her fault, "are far too young to ever have loved someone whom, for reasons of social class or age or circumstance, you could not be with, but I assure you lovers do sometimes face insurmountable obstacles."

"But if they didn't have to part," Viv said, "it would make the ending so much more romantic."

"As I told Trot," Sir Godfrey said dryly, "take it up with the author. Again. From the beginning. We are going to get this right if it kills me. Which it may very well do. Unless the Luftwaffe gets me first." He looked up at the ceiling. "The raids seem rather excessive tonight."

They did, but they began and ended when they were supposed to and hit the correct targets, and there was nothing in Sir Godfrey's Times the next night about security breaches or captured spies, though Mike hadn't phoned again.

Tuesday there was a letter for Eileen. "Is it from Mike?" Polly asked. Perhaps he had decided to write instead of phoning.