Space aliens led by an entity which takes the form of a deceased WW2 officer acquaintance of Nelson's invade the Seaview, intending to reactivate an underwater atomic base to acquire its energy.

A scientist in an underwater sea lab has created a new form of plant life. His twin brother, who can control him telepathically, is on his way there via the Seaview, with plans of using the new plant creatures for his own sinister purposes.