For Marshal O'Neil (Sean Connery), the Jupiter moon Io is just another dingy mining town on the final frontier. When his wife leaves him and takes their son with them, it merely confirms that though he's traveled millions of miles, his life is going nowhere.
Then he notices that miners are dying in strangely psychotic ways -- walking in space without spacesuits, carving up prostitutes. With the help of Dr. Lazarus (Frances Sternhagen), his investigation reveals that the miners are ingesting a lethal drug that speeds their work efforts. He learns that the company that runs the town is behind the drug.
He confronts the town boss Sheppard Peter Boyle and soon has two hit men heading toward Io with a plan to kill him. As the seconds tick down to the next space shuttle's arrival, O'Neil plots to meet them and faces the biggest challenge of his life.