Escaping from a troubled home and struggling to survive on the streets, the abandoned tunnels of the London Underground are a perfect sanctuary for Kai. Along with other teenagers running from their pasts, he finds somewhere to belong in this strange community of outcasts.

But Kai is now facing a very different kind of fight. Every night, led by the enigmatic Spartacus, the runaways must become cage fighters, each fight broadcast to the outside world via YouTube. With pay-per-view profits racking up, Kai and his friends hope to be able to start a new life.

Yet treachery and danger are never far behind, and a new arrival threatens the order that Spartacus has worked so carefully to maintain. And then there is the looming finale, the last battle between Kai and his nemesis Leo: the Kill Fight . . .