Don and Dr Smith are installing some equipment, when Dr Smith comes across and alien prison escapee. The prisoner alien makes Dr Smith change jackets and then runs away. When the prison guards catch up with Dr Smith and Don, they arrest them for aiding an escape of a prisoner.
After a trial, which Maureen, Judy, John and Judy are not present due to being away setting up equipment elsewhere, West and Dr Smith are sentenced to a scorching hot prison planet. They are imprisoned with the prisoner they "helped" escape. The prisoner tries to convince them to help him escape, and they can all share in the reward of deautroniam he has stolen. Dr Smith agrees but Don refuses, so the alien knocks him out.
It turns out the prison guard only arrested West and Dr Smith because he wanted to re-capture the stolen deutronium, and when the alien prisoner is captured once again, and caught, the prison guard thanks Don and Dr Smith for their help, and gives them their freedom back and a few rewards.