They both turned as a middle-aged couple hurtled past, the man muttering, 'Christ!' as he pushed a luggage trolley behind his wife, who was struggling to run in heels, her painted fingernails gripping a bundle of documents. The man glanced across at Katie. Even when rushing for planes, even when their wives were at their sides, men couldn't help but look. They were drawn to her like bees to a honey pot, or like flies to shit as Mia had once said in anger. It wasn't just Katie's petite figure or honey-blonde hair, it was a warm confidence that breathed through her pores, saying, I know who I am.

Katie didn't notice the admiring glance as her attention had been caught by someone else. Finn came loping towards them wearing his daily uniform of T-shirt, jeans and Converse trainers. A tattered army-green backpack hung easily off one shoulder.

Katie took a slight step backwards, aligning herself with Mia, and fed her hands deep into her pockets.

Finn's gaze moved slowly over them both. Then the corners of his mouth turned up in an easy, wide smile. 'The Greene sisters!' If there was any awkwardness on his part, he didn't show it. 'Coming with us, Katie?'

'I'll be living the trip vicariously from all the emails Mia will be sending.'

Mia smiled. 'Hint duly noted.'

An airport vehicle towing a row of luggage trolleys beeped as it rolled towards them, causing the three of them to bunch together.

'So how are things?' Finn asked Katie. 'It's been a while.'

'Yes, it has. Everything is fine, thank you. Work's busy. But good. And you? How are you?'

'Feeling pretty pleased about having a year off.'

'You both must be. It's California first?'

'Yes, for a few weeks of coast-side cruising, and then on to Australia.'

'Sounds wonderful. I'm incredibly jealous.'

Is she? Mia wondered. Would she want this: wearing her life on her back and moving from place to place with no plans?

'Right,' Katie said, taking the car keys from her handbag. 'I best get going.' She glanced at Finn, her face turning serious. 'You will look after her, won't you?'

'You know that's like asking a goldfish to babysit a piranha.'

Her features softened a little. 'Just bring her back safely.'

'I promise.' He leant forward and kissed her on the cheek. 'Take care.'

She nodded quickly, pressing her lips together. 'You'll call?' she said to Mia. 'You've got your mobile?'

'I'm not taking it.' Then, seeing Katie's expression, she added, 'It's too expensive abroad.' But cost wasn't the real reason: Mia didn't want to be contactable.

'I've got mine if you need us,' Finn said. 'You've got my number still?'

'Yes. Yes, I think so.'

There was a brief silence between them all. Mia wondered what Katie would do with the rest of her day. Catch up with a friend over coffee? Go to the gym? Meet Ed for lunch? She realized she had no idea how her sister spent her time.

'Can you let me know when you've arrived?'

'Sure,' Mia replied, with a shrug she hadn't intended. She wanted to tell Katie that she loved her, or say how much she'd miss her, but somehow she couldn't find the words. It had always been that way for her. Instead, she lifted a hand in wave, then turned and left with Finn.

Pressing her nose against the window, she watched London disappearing beneath the white wings of the plane. They rose through a layer of cloud and suddenly the view was swallowed. She sank back in her seat, her heart rate gradually slowing. She had left.

On her lap rested her travel journal. She'd bought it at Camden Market from a stall that sold weathervanes, maps and antique pocket watches. She'd been drawn to the sea-blue fabric that bound the cover and the thick cream pages that smelt like promises.

She opened it, clicked her pen against her collarbone, and wrote her first two lines.

People go travelling for two reasons: because they are searching for something, or because they are running from something. For me, it's both.

She tucked the journal into the seat pocket alongside the laminated flight-safety procedures, and then closed her eyes.

As the plane descended over the Sierra Nevada range, Mia gazed at the clouds drifting below. They looked soft and inviting, and she imagined diving into them, being caught in their fleecy hold and floating with the air currents.

'Not as comfy as they look,' Finn said, as if reading her mind.

Finn Adam Tyler was her best friend and had been since they'd met aged 11 on the school bus. Four weeks ago she'd called him at work to tell him she was going travelling. She was sitting on the kitchen worktop, her heels dangling against the fridge door. When he answered, she said only, 'I've got a plan.'

'What do I need?' he'd replied, a throwback to their teenage years when a plan, if conceived by one of them, had to be adhered to by the other.

She grinned. 'Your passport, a resignation letter, a backpack and a typhoid jab.'

There was a pause. Then, 'Mia, what have you done?'

'Reserved two round-the-world tickets: America, Australia, New Zealand, Fiji, Samoa, Vietnam and Cambodia. The flights leave in four weeks. You coming?'

There was silence. It had hung between them long enough for her to wonder whether her impulsiveness had been a mistake, whether he'd say of course he couldn't just up and leave his job.

'So this typhoid jab,' he'd said eventually, 'is it in the arm or the arse?'

She looked at Finn now: his knees were pushed against the seat in front, a newspaper spread on his lap. The mousey curls of the schoolboy she'd known had now been cut short and rough stubble shadowed his chin.

At the end of their row a voluptuous woman with dangling gold earrings unclipped her seat belt and stepped into the aisle. She moved towards the toilets, gripping the backs of headrests for balance. Mia turned to Finn. 'I need to talk to you.'

'If it's about that last meal, I swear, I thought you wouldn't want to be disturbed.'

She smiled. 'It's something important.'

Finn folded the newspaper over and gave her his full attention.

A few rows in front the faint grizzling of a toddler started up.

Mia tucked her hands beneath her thighs. 'This may sound odd,' she began uncertainly, 'but after I booked our tickets, I realized that there was another place I needed to visit on this trip.' She should have talked to Finn about it sooner, only she was afraid to voice the idea in case she set in motion something she wasn't ready for. Sometimes she wasn't aware that an idea was brewing until it suddenly popped into her mind and she acted upon it. 'I've booked us an extra stop.'

'What?'

'After San Francisco, we've got a flight to Maui.'

'Maui?' He looked blank. 'Why?'

'It's where Mick lives.'

She waited a beat for him to place the name. It had been a long time since he'd heard it.

'Your dad?'

She nodded.

The grizzling child had found its stride and a captive audience; the crying grew louder and something was tossed into the aisle.

Finn was staring at her. 'You haven't talked about him in years. You want to see him?'

'I think so. Yes.'

'Has he ... have you been in contact?'

She shook her head. 'No. Neither of us.' Mick had left when she and Katie were young children, leaving their mother to bring up her two daughters alone.

'I don't understand. Why now?'

It was a fair question, but one she wasn't sure how to answer just yet. She shrugged. Ahead, she heard a taut whisper from the toddler's parent: 'That. Is. Enough.'

Finn ran the knuckle of his thumb under his chin, a habitual gesture when something was worrying him. 'What does Katie think?'

'I haven't told her.'

She could see Finn's surprise and sensed he wanted to say more, but Mia turned to the window, ending the conversation.

She willed her thoughts to drift away with the clouds, knowing it wasn't the only thing she was keeping from her sister.

3.

KATIE.

Cornwall/London, March Katie sat pin straight on the church pew, her feet pressed together. Biting sea air crept through the cracks in the stained-glass windows and twisted beneath the heavy oak door. Her fingers were curled around a damp tissue, Ed's hand resting on top. Eighteen months earlier had seen her seated in this same pew when they buried her mother, only then it had been Mia's fingers linked through her own.

Her gaze was fixed on the coffin. Everything about it the polished shine to the elm wood, the brass clasps keeping it sealed, the white lilies arranged on top suddenly looked wrong. Why had she chosen to bury Mia beside their mother, when her sister had never once visited the grave? Wouldn't cremation have been more suitable, her ashes dispersing on a breeze over a wild sea? Why don't I know what you'd have wanted?

It would have been almost impossible to conceive that Mia was inside the coffin had Katie not decided, two days ago, that she needed to see the body. Ed had been cautious on her behalf. 'Are you sure? We don't know how she may look after the fall.' That's what people were referring to it as: the fall, as if Mia had no more than slipped in the shower, or toppled off a stool.

She wouldn't be dissuaded. Seeing Mia's body would be agony, but to not see it would leave her with the smallest fraction of doubt and if she allowed that doubt to grow over time to hope, she'd be in danger of deluding herself.

When Katie had stepped behind the heavy purple drape in the funeral parlour, she could have fooled herself that Mia was merely sleeping. Her willowy figure, the sweep of dark hair, the curve of her lips, looked as they always had. Yet the proof of death lay in Mia's skin. After months of travelling she would have been deeply tanned, but death had left behind its ghostly pallor so that her skin appeared a strange insipid shade, like milk spilt over a dark floor.

The funeral director had asked if Katie wished to choose an outfit for Mia to be buried in, but she had said no. It had seemed presumptuous to dress Mia, for whom fashion was something indefinable. She fell in love with clothes for their story, choosing a loose shift dress in a deep blue that reminded her of the sea, or picking a second-hand pair of heels because she liked to imagine the places they'd already walked.

On the night Mia died she had been wearing a pair of teal shorts. They had been arranged too high up her waist, not slung low over her hips as she would have worn them. Her feet were bare, a silver toe ring on each foot, her nails unpainted. On her top half she was wearing a cream vest over a turquoise string bikini. A delicate necklace strung with tiny white shells rested at her throat, a single pearl at its centre. She looked too casual for death.

Katie had reached out and placed her hand on Mia's forearm. It felt cold and leaden beneath her fingertips. Slowly, she traced her fingers towards Mia's inner elbow where thin blue veins criss-crossed, no longer carrying blood around her body. She drew her hand over the ridge of Mia's bicep, across her shoulder and along the smooth skin at the nape of her neck. She brushed the faint scar on her temple, a silver crescent, and then her palm rested finally against Mia's cheek. She knew the back of Mia's skull had been cracked open on impact, but there were no other marks on her body. Katie was disappointed: she had been hoping for a clue, something the authorities had missed that would prove Mia had died for a reason more bearable than suicide.

Carefully, she untucked Mia's vest and rearranged her shorts so they rested on her hip bones. Then she leant close to her ear. Her sister's skin smelt unfamiliar: antiseptic and embalming lotion. She closed her eyes as she whispered, 'I am so sorry.'

'Katie?' Ed was squeezing her hand, pulling her thoughts back to the funeral. 'It's you, now.'

He moved his hand to her elbow and helped her stand. Her legs felt light and insubstantial as she left the pew and drifted towards the lectern like a spectre. She tucked her tissue into her coat pocket and pulled from the other a square piece of card on which she'd noted a few sentences.

She glanced up. The church was full. People were standing three deep at the back. She saw old neighbours, friends of Mia's from her schooldays, a group of Katie's girlfriends who'd made the long journey from London. There were many people she didn't recognize, too. A girl in a black woollen hat sobbed quietly, her shoulders shaking. Two rows back, a thin young man blew his nose into a yellow handkerchief and then tucked it beneath his order of service. She knew that the circumstances of Mia's death would be lingering in everyone's thoughts, but she didn't have the answers to address their questions. How could she when she didn't know what to believe herself?

Katie gripped the lectern, cleared her throat twice, and then began. 'While the authorities have made a grey area of Mia's death, her life was a rainbow of colour. As a sister, Mia was dazzling indigo, challenging me to look at the world from new perspectives and see its different shades. She was also the deep violet that drove all her actions straight from her heart, which made her passionate, spontaneous and brave. As a friend she was vibrant orange, spirited, plucky and on the lookout for adventure. As a daughter, I think our mum-' she struggled on that last word. Closing her eyes, she focused on swallowing the rising lump of emotion.

When she opened them, she could see Ed nodding at her, encouraging her on. She took a deep breath and began the sentence again. 'As a daughter, I think our mum would have said Mia was love red, as she filled her with happiness, warmth and laughter. She was also the sea green of the ocean, in which she spent her childhood splashing and tumbling through waves. Her laughter infectious, giddy and frequent was brilliant yellow, a beam of sunlight falling on whoever she laughed with. And now that Mia has gone, for me only cool, empty blue remains in the space where her rainbow once danced.'

Katie left the card on the lectern and somehow her legs carried her back to Ed's side.

The coffin had been lowered into the ground and the funeral party were returning to their cars when Katie saw him.

Finn looked different from the man she'd said goodbye to at the airport. His usually fair skin was bronzed, his hair lightened by the sun to a golden brown, and he looked older, too, having lost the boyish softness in his cheeks. Finn's family had been unable to get in contact with him until three days ago. He had boarded the first flight back to London and arrived yesterday. Flanked by two of his brothers, he glanced up and saw her. His eyes were bloodshot and the skin around his nose was red raw. He moved towards her warily.

'Katie-' he said, but faltered when he saw her expression.

Her voice came out as cold and flat as the sky. 'You left her, Finn.'

He closed his eyes and swallowed. She saw that his lashes were damp. Beyond them a car door slammed and an engine started.

Katie was standing with her back to the stone archway at the rear of the church. She thrust her hands deep into her coat pockets. 'You were supposed to be travelling together. What happened?'

The question seemed painful for him and he looked beyond her as he answered. 'We had an argument. It should never have happened. Mia didn't want to be in Australia-'

'So she went to Bali,' Katie finished. 'Why?'

Finn's left foot, in an unpolished black shoe, jigged up and down. She remembered the gesture; she'd once thought it was a mark of impatience but later understood it to be a sign of nervousness. 'We'd met people who were going out there.'

'I just don't understand any of it.' Katie's hands were beginning to tremble in her pockets. She balled them into fists and lifted her chin. 'Why was she on that cliff top?'

'I don't know. We didn't speak after Australia. She emailed once-'

'You didn't think to tell anyone?' Her voice was growing louder and she was aware of glances being exchanged between Finn's brothers who were hanging back.