Working undercover is all about trust - getting the target to trust you and then betraying them in order to bring them to justice. But what do you do when you believe an undercover cop has crossed the line and aligned herself with the international drugs smuggler she was supposed to be targeting? When Lisa Wilson stops passing on intelligence about her target, MI5 sends in Dan 'Spider' Shepherd to check that she's on the straight and narrow. Now the lives of two undercover operatives are on the line.