They kissed goodbye, and as her mum headed toward the elevator with Sophie, Lily watched them go, noticing how much narrower her mum’s shoulders were than they had been just six months ago.

Back inside her office, Lily sank into the chair at her desk and spun it around to face the window. Fat raindrops hit the glass, sliding down to collect in small pools on the black metal ledge. She breathed slowly and methodically, silently repeating the line her therapist had been schooling her in for years: You can’t change others’ behavior. You can only control your own reaction to it.

Lily closed her eyes, thinking about all the times over the years that Jess’s behavior had tested her reactions. All the times her mum had canceled arrangements because of some eleventh-hour emergency of Jess’s. All the extra help her mum had given Jess because she was a single parent who’d messed up her life. And not once had her mum acknowledged the irony that Jess—who had torn their family apart—was awarded the lion’s share of maternal time and attention.

She thought about her mum, just a few minutes earlier, sitting on the sofa holding her hand, and suddenly found herself imagining the gaping absence her death would leave behind. It was as though something were pressing down hard on her windpipe: an assault, a compression, panic inhaled with each breath.

She was forty-three years old, she told herself. She must have known this day would come eventually, that there would likely be years—decades—when she would be alive and her mum would not. She knew that most children, at some point, became orphans.

Opening her eyes, she looked out of the rain-streaked window to the street ten floors below: miniature figures hurried along the pavement beneath a canopy of umbrellas; car headlights illuminated the glistening tarmac; farther down the river the London Eye continued its almost imperceptible rotation.

Lily knew there was only one person in the world who might understand how she felt about the prospect of losing her mum, just one person who might comprehend how great a loss it would be. But that particular conversation was impossible. These days, she couldn’t even imagine how it might feel.

Lily swung her chair around and pulled it close to her desk, tucking her legs neatly underneath. Focusing on the empty presentation in front of her she began to type, but as her fingers tapped at the keyboard she couldn’t help noticing that her hands were shaking.

Eleven hours later Lily woke with a jolt and sat bolt upright in bed, her heart pounding, beads of sweat gathered at her temples. She looked around the darkened room and glanced at the clock beside her bed—3:04 a.m.—a voice in her head reminding her that it was just a dream.

Switching on the bedside light, she reached across for a glass of water and wrapped her clammy palms around it, sipping gently, her mouth desert-dry. She was desperate to lie down and sleep but every time she blinked, they were there: those tiny blue hummingbirds.