When scientists exploring the Amazon River stumble on a “missing link” connecting humans and fish, they plan to capture it for later study. But the Creature has plans of his own, and has set his sights on the lead scientist's beautiful fiancée, Kay.

Lorelei Lee is a beautiful showgirl engaged to be married to the wealthy Gus Esmond, much to the disapproval of Gus' rich father, Esmond Sr., who thinks that Lorelei is just after his money. When Lorelei goes on a cruise accompanied only by her best friend, Dorothy Shaw, Esmond Sr. hires Ernie Malone, a private detective, to follow her and report any questionable behavior that would disqualify her from the marriage.

The first manned flight to Mars returns after having been out of communications since it had arrived on Mars. What would it reveal?