Dr. Mortimer Gangreen (John Astin) escapes from prison and has set up a base at his assistant Igor's 'Really Big Castle' located outside Paris, France, and he is still bent on global domination. Using his Killer Tomato...展开es, Gangreen plans to stage a second French revolution according to an old prophecy written by Nicodemus, in which the king of France (Louis XVII) will return to claim the throne. The only image of Louis XVII shows that he bears a close resemblance to Igor (Steve Lundquist), and Gangreen plans to use this to his advantage to have Igor impersonate Louis XVII to claim the throne.
The heroes of the film are a has-been TV actor named Michael (Marc Price) and Marie, the French girl he meets and falls in love with (Angela Visser).
As the Killer Tomatoes begin their attack on France, hoping to reach the outskirts of Paris, the true King returns, and faces a showdown with Igor as to who shall rightly become king. With the help of Michael and Marie, Louis triumphs, and Gangreen escapes in a tomato-shaped hot-air balloon, planning revenge and swearing to return in the next movie.