The waters surrounding an island become contaminated by chemical dumping, and people who eat fish caught in those waters become deformed and violent.
| Actor | Role |
|---|---|
 Ian BannenDr. Shaw | Dr. Shaw |
 Judy GeesonVictoria Brown | Victoria Brown |
 John PaulDr. Quist | Dr. Quist |
 George SandersThe Admiral | The Admiral |
 Simon OatesDr. Ridge | Dr. Ridge |
 Jean TrendDr. Fay Chantry | Dr. Fay Chantry |
| JJoby BlanshardBradley | Bradley |
 Percy HerbertHartwell | Hartwell |
 Shelagh FraserMrs. Betty Straker | Mrs. Betty Straker |
 Geoffrey KeenSir Henry Leyton | Sir Henry Leyton |
 Joseph O'ConorVicar | Vicar |
 Norman BirdBrewer | Brewer |
 Constance ChapmanMiss Johnson | Miss Johnson |
 Michael BrennanTom Straker | Tom Straker |
 James CosmoBob Gillette | Bob Gillette |
| CCyril CrossGeorge | George |
 George WoodbridgeFerry Skipper | Ferry Skipper |
 Jerome WillisLt. Com. Tavener | Lt. Com. Tavener |
 Jeremy ChildDavid Broome | David Broome |
| BBrian AnthonyBrian Murray | Brian Murray |
 Rita DaviesMrs. Murray | Mrs. Murray |
| WWalter TurnerMr. Murray | Mr. Murray |
| PPaddy RyanGrandfather Murray | Grandfather Murray |
| RReg LeverSam | Sam |
 Pam St. ClementYoung Woman | Young Woman |