Biologist Ted Baxter (Kent Taylor) and Federal Agent William Grant (Rodney Bell) discover the man's body, washed up on the beach and covered with radiation burns.

Ted eventually discovers that Dr King (Michael Whalen), another marine biologist, created the monster and the radioactive rock with a mutating device he invented in his laboratory.

Ted, Grant, and Lois arrive just in time to witness the large explosion, which destroys the rock, the monster, and Dr King.

[4] ARC's cofounder James H. Nicholson came up with the film's title, as he was looking for a feature to support and team with Day the World Ended.

ARC lacked money to make both, so they allocated Phantom to Dan and Jack Milner, film editors who wanted to get into feature-film production.

In his review, he wrote, "Lots of spy stuff and a lousy monster fail to enliven this deadly dull early American International effort".