Zero Tolerance is an FPS developed by Technopop and published by Accolade in 1994 for the Sega Mega Drive / Genesis console.
The Europa-1 space warship has been attacked by aliens, killing or mutating all aboard. The Planet Defense Corps sends in an elite squad to take back the ship.
The game has 40 levels teeming with aliens and mutants, and you are supposed to kill every one of them. It is possible to rush to the elevator and follow to the next level, but you won't get any more passwords until you go back and kill the remaining enemies.
Zero Tolerance was possibly the first console game (not counting handhelds) to ever allow networking. It required two consoles, two copies of the game, and a special cable that had to be requested from the publisher.
Beyond Zero Tolerance would take the squad to the aliens' homeworld. However, it was cancelled.
After the developer closed doors, its owner released the ROMs for Zero Tolerance and the unfinished sequel as free downloads.