The Doctor and Jo visit the Master in his high-security prison on an island off the south coast of England. The governor, Colonel Trenchard, says ships have been disappearing mysteriously at sea. The Doctor discovers that Trenchard and the Master are in league to contact the Sea Devils, a race of reptiles in hibernation in a base beneath the sea, who have been awoken by recent work on a nearby sea fort. The Master intends to use his new allies to help him conquer the world!
| Actor | Role |
|---|---|
 Jon PertweeThe Doctor | The Doctor |
 Roger DelgadoThe Master | The Master |
 Katy ManningJo Grant | Jo Grant |
 Edwin RichfieldCaptain Hart | Captain Hart |
 Clive MortonTrenchard | Trenchard |
| JJune Murphy3rd Officer Jane Blythe | 3rd Officer Jane Blythe |
 Donald SumpterCommander Ridgeway | Commander Ridgeway |
| DDavid GriffinLt. Commander Mitchell | Lt. Commander Mitchell |
| HHugh FutcherHickman | Hickman |
 Royston TicknerRobbins | Robbins |
 Pat GormanSea Devil | Sea Devil |
 Peter Forbes-RobertsonChief Sea Devil | Chief Sea Devil |
| DDeclan MulhollandClark | Clark |
| AAlec WallisLdg. Telegraphist Bowman | Ldg. Telegraphist Bowman |
| BBrian JusticeCastle Guard Wilson | Castle Guard Wilson |
 Terry WalshCastle Guard Barclay | Castle Guard Barclay |
| EEric MasonC.P.O. Smedley | C.P.O. Smedley |
| CChristopher WrayLdg. Seaman Lovell | Ldg. Seaman Lovell |
| SStanley McGeaghCastle Guard Drew | Castle Guard Drew |
| BBrian VaughanLt. Commander Watts | Lt. Commander Watts |
 Martin BoddeyWalker | Walker |