They encounter an undersea flying saucer, after which the lab is attacked by a colorful aquatic humanoid monster who they fear may be the first in an alien invasion.

US Navy Commander Wayne has arrived at Topside, the support vessel for the civilian Institute of Marine Sciences' Sealab, a facility on the ocean floor.

Wayne rides a diving bell down to Sealab, where he meets its director, Dr. LaSaltier and marine biologist Dr. Rene Peron.

Wayne's arrival, a minisub carrying diver Hugh Maddox and photographer Sandra Welles is approaching the submerged object so as to get a clear photo of it.

The don gasmasks and go inside with fire extinguishers to disperse the vapor, only to discover that the cylinder has burst.

Wayne lures the monster into a trap he has built - several spearguns set to fire when it triggers tripwires.

Wayne and Sandra to flee, then dies a heroic death in the explosion that obliterates the saucer and kills the monster.

But the Commander sets him straight, telling him they have proven life exists on other planets and that we must learn how to communicate with extraterrestrials.

United Picture Corporation's first two films, Castle of Evil and Destination Inner Space, were shot back-to-back in only 14 days.

[4] Principal photography of Destination Inner Space began on November 17, 1965 at Producers Studio in Los Angeles.

Its production was financed in part by its having been licensed in advance to 44 TV stations by Television Enterprises Corporation (TEC).

After roles in a number of features, she had been working extensively in TV and on stage, but had not appeared in a motion picture since Mardi Gras (1958).

[6] However, film critic Bruce Elder refers to the music in Destination Inner Space as "mostly hand-me-down work" from The Angry Red Planet.

[citation needed] May 2024, KL Studio Classics released a high definition print of Destination Inner Space on Blu ray, as part of the Sci-Fi Chillers Collection.

[8] His final assessment is, "Low budget and average performances do not prevent director Francis Lyon from providing a first-rate entertainment.