On Jupiter's moon Ganymede, an alien life-form has taken over the body of a human astronaut, creating an almost perfect copy. It then begins the long return journey to Earth.
| Actor | Role |
|---|---|
 Alexander DavionDr. Crawford | Dr. Crawford |
 David HemmingsRoger Westcott | Roger Westcott |
 Charles TingwellCaptain Jaffe | Captain Jaffe |
| PPeter FraserDonnie Chaffer | Donnie Chaffer |
 Tony WagerJensen | Jensen |
 Keith BuckleyScotty McIntyre | Scotty McIntyre |
| DDavid SavileGerry | Gerry |
| GGeoffrey KenionDave | Dave |
| BBarry AshtonFrank | Frank |
| DDavid MunroKen | Ken |
 Hedger WallaceCommander | Commander |
| LLew LutonOfficer | Officer |
 Derek MartinGuard | Guard |