The acclaimed feature film of the BBC TV series in which Dr Del Shaw (Ian Bannen) of Doomwatch - the British government's environmental monitoring organisation - travels to the island of Balfe to investigate pollution.
A year earlier an oil tanker was holed and its cargo contaminated the local waters - but has there been any adverse long-term effects? The villagers are all suspicious of Shaw, especially the menfolk, who seem to have transformed into near-neanderthals.
Samples of the local fish sent back to Doomwatch prove to be unnaturally large, and after teaming up with a local teacher (Judy Geeson) Shaw discovers that the transformation of the men is due to a rare disease called acromelagy. Co-stars Percy Herbert and George Sanders.