Polly, 28, lives in London with a "perfect-on-paper" boyfriend and a dead-end job. Her banal existence is turned upside down late one drunken night after a chance encounter with a man on a packed train.

The chemistry between them is electric and on impulse, they kiss, giving in to their carnal desires. But it’s over in an instant, and Polly is left shell-shocked as he walks away without even telling her his name.

Polly begins a frantic online search, and discovers more about "Tubing," an underground phenomenon in which total strangers set up illicit, silent, sexual meetings on busy commuter tube trains. In the process, she tracks him down and he slowly lures her into his murky world, setting up encounters with different men via Twitter.

At first she thinks she can keep it separate from the rest of her life, but things soon spiral out of control. A horrific turn of events make Polly realize not only how foolish she has been, but how much danger she is in. Can she get out before it’s too late?