In this serial, the Doctor (Patrick Troughton) and his travelling companions Jamie McCrimmon (Frazer Hines) and Victoria Waterfield (Deborah Watling) find themselves at a Euro Sea Gas refinery off the coast of contemporary England, where an infectious weed spreads rapidly, infecting and mind controlling anyone who comes into contact with it, including the refinery leader Robson (Victor Maddern). This story is the last to feature Watling as Victoria Waterfield. It also marks the first appearance of the Doctor's gadget, the sonic screwdriver. Although audio recordings, still photographs, and clips of the story exist, no episodes of this serial are known to have survived. Chronologically, it is the last story to be completely missing from the BBC Archives.