'XL5 to H20' by Alan Fennell

A vicious Aquaphibian attacks the inhabitants of the planet Zolpheid, killing everyone except for two survivors, Rald and Jerek. When Space City receives the Zolpheids’ distress call, Commander Zero instructs Fireball XL5 to rescue them. However, when Steve and Matt take a specially -reinforced Fireball Junior into the watery depths of the planet, they come face-to-face with the Aquaphibian and its deadly gas gun…

Paul Maxwell (Colonel Steve Zodiac), Sylvia Anderson (Doctor Venus), David Graham (Professor Matt Matic / Lieutenant 90 / Reld), John Bluthal (Commander Zero / Jenek), Gerry Anderson (Robert the Robot)

Directed by John Kelly
Produced by Gerry Anderson
Associate Producer Reg Hill

TX (UK - ITV):
6th May 1963

Notes:
*Featuring Colonel Steve Zodiac, Venus, Professor Matt Matic, Commander Zero, Lieutenant 90 and Robert the Robot