There’s an enemy among you. Someone in the group is a traitor, working for another organisation (like Torchwood) or for an alien power (like the Master or the Krillitane). This traitor is undercover, hiding their secret goals from you. One day in the future, when the time is right, they will betray you.

The GM determines who the traitor is. It could be your superior in the organisation, or a trusted underling. It could even be one of the player characters!