When strangers take part in a series of group suicides, everything suggests that a cult is to blame. But how do you stop a cult when no one knows they are members?

Nine people arrive one night on Chelsea Bridge. They’ve never met. But at the same time, they run and leap to their deaths. Each of them received a letter in the post that morning, a pre-written suicide note, and a page containing only four words: Nothing important happened today.

That is how they knew they had been chosen to become a part of the People of Choice: a mysterious suicide cult whose members have no knowledge of one another.

32 people on that train witness the event. Two of them will be next. By the morning, People of Choice are appearing around the globe: a decapitation in Germany, a public shooting at a university in Bordeaux; in Illinois, a sports team stands around the centre circle of the football pitch and pulls the trigger of the gun pressed to the temple of the person on their right. It becomes a movement.

A social media page that has lain dormant for four years suddenly has thousands of followers. The police are under pressure to find a link between the cult members, to locate a leader that does not seem to exist.

But how do you stop a cult when people do not know they are members?

One week after the two witnesses jump to their deaths, 20 letters are opened across London that all say the same thing. Later that day, the strangers all know to meet at Tower Bridge. The Teacher will jump first. The Detective will be last....