Naval ships have been disappearing, and Seaview is on a hunt for the undersea installation responsible. They find a sea-lab that is not only well-armed but surrounded by overgrown and ferocious sea-creatures. To complicate matters, the guest expert aboard is a traitor. Various "dangerous denizens" of the deep from earlier episodes reappear here, thanks to re-used footage: there's a whale, and one of those globular-eyed bottom-dwelling monsters seen in such episodes as "Deadly Creature Below".