The Doctor, Tegan, and Turlough are drawn off course by a mysterious time corridor linking 20th-century London to a derelict space station in the far future. On Earth, a police squad investigates brutal murders near the corridor’s exit. Meanwhile, on the space station, a Dalek task force prepares to break through quarantine and free their creator Davros from cryogenic suspension. The Doctor decides to investigate the corridor’s origin — and narrowly avoids a Dalek ambush.