When a West German police officer defects to the GDR, he brings news of a mole in the Stasi. Lieutenant Reim leads the initial inquiry but is unconvinced by the defector's claims. As news of the affair spreads through Berlin Centre, Reim is sent to Bonn on a mission to catch a mole he doesn't want to find - does his reluctance to investigate have anything to do with the secrets he'd prefer remained hidden?