On Tuesday morning, Merry found herself alone at last as her parents and Gawain headed off to Brecon to see if the antiques dealer had managed to sell the old silver photo frames and set of chairs they’d taken to him the previous week.

In the ringing silence of the farmhouse, she thought over what had happened in the riddle pool. She had been lucky to survive, she knew that. She had tried to swim through to whatever wonders lay hidden and she had failed. Spectacularly.

But there were two responses to failure: give up, or try harder. The first was not in her nature. Time for a new plan . . .

She rose to her feet in one quick, smooth movement. She made a phone call, loaded a backpack, pulled on her waterproof jacket and boots and set out.

She took her knife. She thought she’d be safe now, but she still liked to have it on her. The press announcement had gone out as promised. She’d seen no sign of anyone suspicious. Hadn’t felt as if anyone was watching her . . . but she couldn’t know for sure. She still stopped, still turned circles, thought it was a new habit she wouldn’t lose.

She got to the Black Castle, to the drawbridge. Crossing the moat, she felt she was walking back hundreds of years.

The courtyard echoed to her steps. Merry paused at the giant door and rapped the lion’s head knocker against the solid oak. Nothing. She opened the door and stepped inside the Great Hall.

‘Mrs Baskerville? It’s Merry,’ she called out, breaking the heavy silence that hung like a living hush, like something holding its breath.

Heavy footsteps echoed up the far staircase. Mrs Baskerville heaved into view.

‘Sorry, bach!’ she panted. Bach was a term of endearment. It meant little one in Welsh. Mrs Baskerville had referred to Merry as bach since she was a baby and saw no reason to change now. She gave Merry a shrewd look. ‘Shouldn’t you be in bed resting?’

‘Any more rest would kill me! I’m fine!’ Merry declared.

The housekeeper spluttered with laughter. ‘Now, what was it you wanted to ask me?’

‘I was wondering if I might use the pool. Today and for the next few weeks.’

‘I’d have thought you’d have had enough of water for the time being.’

‘Not really,’ replied Merry, trying to smile blandly, give nothing away.

The housekeeper blew out a breath. ‘There’s no key. Just a keypad. One zero six six is the code. Whatever you do, don’t drown and don’t get caught. Check with me first in case the earl or countess are in residence.’

‘When are they coming back?’

‘Ten days. With Lady Alicia. Goodness alone knows when Lord James’ll be back,’ she added wistfully.

Merry gave her a sympathetic glance. Mrs Baskerville had known James since he was born.

‘Criminal, if you ask me,’ muttered the housekeeper just loud enough to be heard. ‘Have you spoken to him?’

‘Every day,’ replied Merry.

‘Any news?

‘Not yet,’ replied Merry.

She thanked the housekeeper and headed out again. She thought about James, wondered if he was lonely. Maybe today would bring the news he yearned to hear, the offer from Manchester United that would make it all worthwhile. She crossed her fingers, sent out a silent prayer.

Merry followed the path of stones that led from the castle to the copse of fir trees that hid the swimming pool complex and greenhouse. She reached down, grabbed a couple of handfuls of stones and filled her pockets.

She skirted the copse, inhaling the crisp smell of damp pine, and eyed the big wood-and-glass structure, the vaulted roof. It was huge. She tapped in the code on the control panel, pushed through. The door closed with a soft hiss.

Inside it was deliciously warm. The sweet smell of cedar filled the air. The water filter murmured gently. No reek of bleach and mould, no screaming babies with leaking nappies like in the council pool.

She stripped off in the cosy changing room, removed her eye patch and laid it on her pile of clothes. The stone floor was warm underfoot; low lights cast a golden glow and clean white towels hung on heated rails. She pulled on her swimsuit, then put her jacket back on. She did up the buttons on the pockets so the stones wouldn’t fall out, put on her goggles and walked from the changing room to the edge of the pool.

A shaft of sunlight pierced the clouds and shone through the glass ceiling into the pool, dappling the water. This pool was about as welcoming as water got. But she couldn’t go in. She was breathing fast, adrenalin surging. She wasn’t scared. She was terrified. The memory of what had happened, of her near drowning, had gone deep into her body. This wasn’t a mentally generated fear that she could fight with reason and logic. It was her body’s own fear.

She stood on the side of the gorgeous pool, forcing her breathing to slow. She felt a burn of misery and anger that one of her major pleasures had been turned into terror.

She knew what to do . . . Face your fears; get back on the horse; get back in the water . . . She took a breath and dived . . .

The memories hit her. The dark cave, the low roof, the current, so strong, pushing her as she battled it back. Panic rose and she wanted to open her mouth and gulp. She fought it down but she managed only three strokes underwater before she had to surface, gasping, heart hammering. She tried again but her body, so long something she thought she could control, defied her. It did not want to go under the water. So she stayed on the surface, forced herself to keep swimming. She was like a beginner, gasping, splashing, panic tiring her. She carried on, muscles burning.

Her father had told her there is nothing more tiring than terror. It’s why in the army you have to be super fit. She was super fit. And she was exhausted. She vowed to do twenty lengths before she allowed herself to get out.

Back and forth in the blood-warm water, the heavy jacket weighing her down. She counted off the lengths. At twenty, legs rubbery, she stepped from the pool, walked to the changing room, stood under the power shower. This was water her body could handle. She stood there for a long time, letting the water wash away the memories.

Caradoc, Elinor and Gawain were still out when Merry returned home and she was glad of it. She had time to compose herself, time to veil another set of lies – What have you been up to? Oh, just went for a walk – something innocuous in case she had been seen. After all, the valley had plenty of eyes and mouths.

Merry washed her swimming costume, wrung it out and hung it to dry in her bedroom on the inside of the door, dangling off the handle. She didn’t want an interrogation if her mother found it. She grabbed a couple of Welsh cakes and brewed up a cup of coffee and settled down at the PC.

She googled underwater swimming, picked a web link written by a former US Navy SEAL. If it was good enough for the guys who got Osama Bin Laden, it was good enough for her.

She clicked on a YouTube demo of the SEAL and his underwater technique.

Lesson one: save energy. He used a combination of modified breaststroke with a dolphin kick at the end and a hard downwards arm push rather than a typical breaststroke lateral one. Minimize strokes, he said. Underwater swimming should be no more taxing than walking while holding your breath.

Merry snorted at that one. She had a long way to go.

Lesson two: do not hyperventilate before going under. You CANNOT increase your oxygen levels by taking additional deep breaths before swimming underwater. Instead you are reducing you levels of CO2 and so increasing the danger of shallow-water blackout. Doing strenuous exercise before swimming underwater could also have the same effect because that could make you hyperventilate too. One big inhale/exhale and one last inhale is all you need.

Merry filed that away. Thank you, Google.

There was a last word of warning: Do NOT do this alone. Many people have died trying.

That was the only piece of his advice that Merry did not plan to follow.