Season 3 Episode 21: Circle

"Previously dismissed as harmless quasi-scientific cultists, the mysterious organisation called The Circle is in fact far deadlier than anyone could have predicted. Already responsible for completely changing the course of Carlos' life, they are hell-bent on killing Sarah before she becomes Fran, her future alter-ego. But The Circle have grown complacent - at least according to Claudia Horne, their leader's own daughter. As she stages a coup within The Circle's highest ranks, the London Field Team are meanwhile trapped in 1942, while Sarah must once again judge who is a friend and who is an enemy..."