Brown is returning to Earth after his last successful mission, aboard the cruise ship Space Adventurer. He has been assigned a minder, Marika, by Earth, to stop him going AWOL. During a stop at the space station Outer Reach, Brown tries to meet his old friend Brett. Brown thinks he can supply some insider information about the planet Levita, where Brown has a son, he has never seen. Instead Brett attempts to kidnap him and ship him to Levita, where he is a wanted man. While escaping Brown destroys two super mandroids.
Brown then meets Leela Junkers, who is investigating the destruction of the mandroids, which are manufactured by her father on Outer Reach. Leela seduces Brown, who is certain that Leela knows he is responsible, but Leela suspects Brown is a changeling, so decides to protect him and inform her father otherwise. She disembarks suddenly, without saying goodbye.
There is another attempt to take Brown hostage from the ship, by an alien vessel. Brown foils this attempt and saves Marika in the process. She then becomes attracted to him. Brown finds out from a prisoner, that they wanted his help to stop an invasion of their planet by Earth. They have discovered, using a special analytical tool that he is a highly pivotal person who will influence future events.
Earlier Brown had discovered that he is an alien half-breed and is able to shape-change, into an amphibian. He finds that he is a result of Species X changelings, who have assimilated with humans and are ruling Earth secretly. Leela, also a half-breed and her father Vincent, an original, are two of them.
When Brown discovers that all their lives are threatened by a sinister group he becomes engulfed in a life and death struggle to save them and Earth.