Set in the distant future, Federal Marshal William T. O'Niel (Sean Connery), is assigned to a one-year tour of duty at Con Am 27, a titanium ore mining outpost on Jupiter's third moon Io (pronounced "eye-oh"), a volcanic moon about 2,275 miles in diameter and located 275,000 miles from Jupiter's gas surface. Conditions on Io are difficult - gravity is 1/6th that of Earth with no breathable atmosphere, atmospheric work-suits are cumbersome, and miners carry around their own air supply whilst working. Shifts are long but significant bonuses are paid as incentives to boost production. Explaining in an opening disclaimer, Con Am 27 is franchised to Con-Amalgamate by the League of Industrial Nations located on Earth. The colony's population is 2,144 men and women (1,250 labor, 714 support personnel, and 180 administration and maintenance crew). It is policed by a forced called Administrative Level 4 and above.
Mark Sheppard (Peter Boyle), the head of the mining colony, boasts that since he took over the running of Io productivity has broken all previous records.
The film begins with a miner named Tarlow (John Ratzenberger) suffering an attack of the DT's who rips open his work-suit which then decompresses resulting in his death. Shortly thereafter another miner Cane (Eugene Lipinski) enters a lift to the mine without wearing his work-suit, and likewise dies from decompression.
With the assistance of Dr. Lazarus (Frances Sternhagen), O'Niel starts an investigation into these deaths. However, before he can complete his investigation there is a further incident, this time involving a worker Sagan (Steven Berkoff) threatening to kill a prostitute with a knife. At the scene of this incident O'Niel attempts to calm the man and enter the room by the main door, whilst Montone, O'Niel's sergeant (James B. Sikking), enters the room via the air duct. Montone shoots Sagan dead before O'Niel can talk with him.
O'Niel takes a sample of blood from the dead Sagan in the morgue and has Lazarus examine it in her lab. It is here where they both discover the one thing all the deaths have in common that all of the miners had a lethal amphetamine-type drug, called "polydichloric euthimal" in their bloodstream. The drug allows the miners to work continuously for days at a time until they become "burned out" and turn psychotic. O'Niel uncovers a drug ring to distribute these drugs to the miners, run by Sheppard and effectively sanctioned by Montone.
O'Niel confronts Sheppard, but the administrator smugly informs him that nobody else wants the drug shipments stopped; production is up, the workers are happy, the corporate owners (Consolidated Amalgamated) are also happy, therefore Sheppard is happy. O'Niel vows to expose the entire scheme.
Meanwhile, O'Niel's wife (Kika Markham), frustrated that he considers justice more important than his family, leaves him a message pleading with him to return with her to Earth.
By the use of surveillance cameras, O'Niel pursues and captures one of Sheppard's drug dealers and places him in isolation. However, before he can be questioned by O'Niel the dealer is killed. Montone is then found garroted in his quarters. O'Niel does however manage to obtain, and destroy, the latest consignment of drugs which has been hidden in a food shipment recently arrived from the Jupiter Space Station. When Sheppard is informed of this, he contacts his company rep aboard the orbiting space station about 70 hours away from Io and informs him of the situation and summons two professional hitmen to kill O'Niel, but his message is monitored by O'Niel. Sheppard also tells his unseen contact that he has an "inside man" also ready to kill O'Niel if need be. The company rep warns Sheppard that if his men fail to kill O'Niel, the next assassins the company sends will be coming after Sheppard.
At this point, the film evolves into a 'High Noon' situation as O'Niel waits during the 70 hours it takes from the shuttle to travel from the space station to arrive at Con Am 27. The fact that the shuttle is bearing professional assassins is spread around the facility, and O'Niel's attempts to defend himself by organising resistance fall on deaf ears as no-one is prepared to risk their lives to help him. Aware that he may not survive, O'Niel last's act is to send a message to his wife and son (who left him early in the film) that he will join them on their trip home to Earth.
The shuttle arrives about 42 minutes early. Over the surveillance cameras, O'Niel sees the two assassins, armed with high-tech rapid firing shotguns, separate from the rest of the miners and go hunting for O'Niel. With the sole assistance of Dr. Lazarus, O'Niel engages in a desperate kill-or-be-killed chase through the colony before dispatching the assassins. After being wounded in an ambush by one of the assassins. O'Niel dons a space suit and goes out and kills one of the assassins when Dr. Lazarus traps him in an umbilicus corridor and O'Niel depressurizes it to kill the assassin.
O'Niel dispatches the next assassin who enters the colony's greenhouse and tricks him into shooting at a falling object outside the glass where the greenhouse depressurizes after one of its windows is shot out and the assassin is swept out by the vacuum to his death.
But while still outside, O'Niel comes face-to-face with the last assassin: his very own Deputy Ballard (Clarke Peters), armed with a shotgun, who follows him out and attempts to kill him. O'Niel runs where he turns the tables on his traitorous deputy and engages in hand-to-hand combat after he disarms him. O'Niel pulls out an oxygen tube to the assassin's spacesuit and the deputy too dies from depressurization.
Afterwords, a wounded and exhausted O'Niel confronts Sheppard in the outpost's bar where all the other colony employees have gathered and punches him out. Arrest warrants are issued for all those involved in the drug smuggling operation, including Sheppard. After this, O'Niel's tour of duty is complete and he retires, accompanying his family back on Earth.