At that moment the eye that is on the young woman registers movement. Sondra turns her head and sees that the young woman has jumped down onto the walkway and is dragging herself towards the gap in the fence where the suspension wire attaches to the bridge.

Oh God, thinks Sondra. I shouldn’t have walked away. I shouldn’t have left her. She’s going to go and I’m not going to be able to pull her back. She feels herself take a leap forward and closes her eyes.

Chapter 1

Nevis

Nevis Smith, student mathematician, bird lover, and keeper of secrets, is lying on her bed in the flat she shares with Satnam Mann trying to finish a tricky piece of coursework on deep vent modelling when her phone bleeps with Satnam’s ringtone.

‘Hey, I thought you’d gone to bed already.’ The door to Satnam’s room was shut and the light was out when Nevis came in late from the library.

An unfamiliar voice replies. ‘My name is Sondra. I’m with your friend Satnam on the Clifton Suspension Bridge. She wants you to come straight away. Please come, right now.’

Nevis says, ‘Is she hurt?’

‘Not yet, not yet, but she’s in a bad way. Oh please. Don’t call the emergency services. She’s says that if she sees a blue light she’ll jump. Please, I don’t know what else to say to her, just come.’

The words hum and hiss and swirl around in Nevis’s head. Is this some kind of joke? Or a scam? Someone playing a sick prank. What would Satnam be doing on the bridge past midnight? Why would this woman be calling? In any case what Sondra is saying is impossible because it’s after midnight and Satnam is asleep in her bed.

And yet the urgency of the voice is unmistakeable. Nevis rushes into the hallway and throws open the door to Satnam’s room. In the murky light she picks out the shape of an empty bed. There is something rancid in the air which she has never noticed before. Is she imagining things? She can hear her head drumming or is it her heart? What is happening? Nevis, who prides herself on thinking straight, can hardly think at all.

She reaches for the light switch and flips it on as if that might illuminate the inside of her head. But no. The bed remains empty. She walks around it and opens the wardrobe. She calls, ‘Satnam?’ and hears traffic outside and the clamour of her pulse. What is going on? As she turns to leave, her eye catches a bottle of vodka on the bedside table. How could she have missed it? She hurries over, picks it up, shakes the few remaining drops inside the bottle, puts it back down and feels the moving parts of her brain clicking into place at last.