The opposition believe Steed is responsible for the death of their agent Vinkel. Steed, however, had nothing to do with the murder and, as a show of good faith, agrees to a swap of partners with Keller in a bid to find the real killer.

The Ministry has been broken into by, it seems, an invisible man. A trail leads via inventor Quilby to the Eastern Drug Company, a front organisation run by Soviet intelligence. Meanwhile, the crack spies Elena and Alexandre Vazin are in town. But nobody has seen anything of Alexandre....