Nelson, Crane, and Dr. Land are in an undersea cavern looking for Zycron, a recently discovered and powerful energy source. Their mission is to secure the element and keep it from falling into enemy hands. However, unknown to Crane and Land, Nelson had been brainwashed and ordered to locate the Zycron and destroy Seaview.
- This is the final episode of the third season.
- This episode was permitted the budget to support the full cast, guest actors with speaking lines, and optical effects such as burn in laser beams.
- This is the only episode with "Seaview" in the title.
- When Sharkey is shot in the climax, a squib was used to show the impact of the bullet into his shoulder. This is the only example of a realistic shooting on the series.
- Richard Basehart plays Nelson with a calm reserve, rathern than menacing or evil, as if destroying the sub was part of the normal routine.
- Jerry Catron played Richards in The Fossil Men . He was also Mark Goddard's stunt double.
- Arthur Space (Dr. Land) did not appear on any other Irwin Allen series, however he worked with June Lockhart on Lassie and Guy Williams on Zorro.