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, Kai 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 gambling profits from these videos 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 . . .