Land of the Giants

The Earthlings find another Earthling, an amnesiac who thinks his name is "Joe", on the run from two giants. They take him back to the Spindrift, although Steve isn't happy with the decision. Joe is dressed in a pilot's uniform, but is unable to remember anything about his past.

Steve and Joe get recaptured by the same two giants and taken back to their lair, but manage to escape. The castaways are convinced of Joe's credentials since he helped Steve escape, but Steve is somewhat suspicious at the ease of their escape. Joe angrily defends himself then wanders off, and Betty goes to follow him...only to find out he is in league with the two giants. They have discovered a pill that shrinks giants down to human size, and plan to take the crew captive, learn how to fly the ship, and then take it back to Earth for its advanced technology.

Betty is held prisoner, and Joe convinces the others he knows where some ion propellant is that they need to repair the ship. They get the isotope but Joe gets captured and the rest come back to rescue him. Joe, repenting of his evil ways, confronts his partners (who have captured everyone but Steve). Steve uses the explosive isotope to force the giants to release the others, and Joe takes it and holds his partners at bay long enough for the Earthlings to escape. The building blows up, leaving a dead, restored-to-normal-sized Joe.