Scientist Dr. Cal Meacham (Rex Reason) returns to his home after leaving a conference. Dr. Meacham flies himself but his plane loses power but is fortunately saved by a mysterious green light.
Once he arrives home his assistant Joe Wilson (Robert Nichols) informs him of a mysterious package arriving. The package contains instructions for building a strange new device. Dr. Meacham and Wilson quickly build what turns out to be something called an interocitor, a TV-like device that suddenly has a strange man named Exeter (Jeff Morrow) appear on it informing Meacham that he has passed a test and is invited to join other scientists working for Exeter. Wilson thinks it’s a bad idea but Dr. Meacham goes.
When Meacham arrives he finds his former flame Dr. Ruth Adams (Faith Domergue) working there but she doesn’t seem to recognize him. The other staff are strangely distant and Exeter informs Meacham that he wants them to work on a way to quickly produce nuclear power. Later when Meacham finally gets Adams and another scientist Steve Carlson (Russell Johnson) alone, the two inform Meacham that they are the only scientists who haven’t been brainwashed by Exeter. Exeter’s assistant Brack (Lance Fuller) overhears the conversation and wants Exeter to brainwash the bunch.
The trio try to escape but Steve is killed as Meachman and Adams try to fly away in a helicopter only to get picked up by an alien spaceship. Exeter and his people are aliens from the planet Metaluna and they are at war with a race called Zahgon. Exeter’s planet has taken a beating and they were hoping that the scientists could help them defeat their enemies. Once they get to Metaluna things get worse.
Director Joseph M. Newman never directed a sci-fi movie before. Most of his work had been lower budget crime/film noir efforts. The studio hired uncredited Jack Arnold to direct the last half of the film because of Newman’s inexperience.
Mystery Science Theatre 3000 made fun of this movie for their big screen feature film which cut 20 minutes out of the movie and ended up being shorter than the standard Mystery Science Theatre 3000 TV episode. Personally picking on This Island Earth seemed kind of weak when there are far far worse sci-fi movies out. This Island Earth is actually pretty good. The movie itself is pretty entertaining and the mutant from Metaluna is awesome looking.