“The Man With Two Shadows” hinges on an enemy plot to create doubles of various important people and have the doubles kill and then take over the lives and roles of their originals. Steed and Cathy find out that Steed is one of the people who is going to be replaced, and Steed admits that at one time he had been captured by the bad guys who are making the doubles but that he escaped after four days. They also discover that a man named Gordon who is at the holiday camp where most of the action takes place isn’t really Gordon, but is in fact his double.
After Cathy returns to London to see what Charles, Steed’s supervisor, has come up with, Steed is attacked by his double in an attempted murder that Steed manages to foil; the double is killed instead. But the possibility of Steed being a doppelgänger weighs heavily on Cathy, and by the end of Act II she has become uncertain that Steed is, in fact, real-Steed. She gets orders from Charles to kill Steed if she thinks that he could be the doppelgänger. But Cathy also is unconvinced that Steed is fake-Steed. So she engages in a tour-de-force of logic by which she convinces herself that Steed is who he says he is, and also outs one of the other doubles involved in the plot, a man posing as a member of Parliament named Cummings.