She paused on the first-floor landing. Jack and Alexa’s bedrooms were on the left. The only bathroom was dead ahead of her. Then there was another staircase that twisted back upon itself and ended in the loft. Ellen ran up the uncarpeted staircase into the attic, taking huge gulps of breath. She yanked the door open and took in the scene. Before she could stop herself, she screamed. About a metre or so from the doorway lay a huge, very dead mouse, lying on its back with its paws pointing straight up. She steadied herself and breathed deeply, placing her hand on her chest to slow her hurtling heartbeat, trying to banish the blackness that was seeping into the corner of her eyes.

It was warm in there, and smelled faintly of wood chips and school music cupboards. As her pulse slowed, she peered cautiously into the attic’s chaos. She’d only been up here once, and it still had all the ambience of a pound shop on the Old Kent Road: packed to the eaves with plastic crates of DVDs, warped cardboard boxes full of cables, half-full litre bottles of white spirit, and broken dining-room chairs. In one corner, there was an open suitcase of BB guns and next to it a teetering pile of The Guinness Book of Records, most of them from the 1990s. She couldn’t see anything that might be the mains water switch.

Heart still galloping, Ellen heard a bedroom door open on the landing below, followed by tentative footsteps on the staircase. From the gait, she guessed – correctly, as it turned out – that it was Jack, and rolled her eyes in spite of herself. Jack was not the man you’d pick first in a crisis. Her housemate was about six foot tall and very lanky, with a baby-face and dark hair that was slightly too long. He had the permanent air of an overladen man boarding a train just before it leaves the station and trying to untangle himself from his luggage, carrier bags swinging from every limb. He worked in customer service for a vegetable delivery box start-up, Green Genie, which meant he had access to a bounty of wonky carrots and misshapen strawberries. Occasionally, he would make them extravagant smoothies that always ended up tasting overwhelmingly of grit.

The footsteps had stopped.

‘Hello?’ Jack called hesitantly, from below. ‘Ellen?’

‘I’m up here!’

He didn’t move.

She pressed her lips together and resisted the urge to snap. ‘Jack, can you come here a second?’ she said, civilly. An afterthought. ‘Please?’

She heard his footsteps on the stairs again, slow and deliberate. He appeared at last, wearing a pair of blue tracksuit bottoms and a Green Genie T-shirt. The logo was a dancing carrot. He paused on the penultimate stair, watching her warily.

‘Sorry, Jack. I didn’t mean to wake you. But –’

‘You screamed,’ he interrupted. When he saw the mouse his eyes widened.