The TARDIS brings the Doctor, Jamie and Victoria to the present-day North Sea coast. Something nasty is lurking in the gas pipelines of the North Sea, and before long the nearby refinery is under attack. The Doctor discovers that a form of living seaweed is at work, and that a number of the base personnel have been possessed. Can he and his friends work in time to prevent an entire take-over of Earth?
| Actor | Role |
|---|---|
 Patrick TroughtonThe Doctor | The Doctor |
 Frazer HinesJamie McCrimmon | Jamie McCrimmon |
 Deborah WatlingVictoria Waterfield | Victoria Waterfield |
 Victor MaddernRobson | Robson |
| RRoy SpencerFrank Harris | Frank Harris |
| JJune MurphyMaggie Harris | Maggie Harris |
 John AbineriVan Lutyens | Van Lutyens |
| JJohn GarvinCarney | Carney |
| HHubert ReesChief Engineer | Chief Engineer |
| GGraham LeamanPrice | Price |
| RRichard MayesChief Baxter | Chief Baxter |
 Margaret JohnMegan Jones | Megan Jones |
| BBrian CullingfordPerkins | Perkins |
| BBill BurridgeMr. Quill | Mr. Quill |
| PPeter DucrowGuard | Guard |
| JJohn GillMr. Oak | Mr. Oak |