Zero Tolerance has not aged well as Doom's gameplay raises player expectations of FPSs but it is still worth a play.
The plot isn't worth your attention, but essentially boils down to a team of specialist soldier taking back a space station, Europe 1, from an alien attack.
Your job is to shoot, burn and blow up each and every one of them, floor by floor, something the text seems to imply the team do routinely.
Well get the aspirin out. This is a game that tries hard to entertain and innovation, but ultimately just brings on a headache. Rooms are fairly large, and dark and enemies can get shots off at you before they can be seen, are considerably faster than you. Their AI consists of running quickly behind you and shooting you in the back while you are screaming at the joypad's D pad to allow you to turn on the spot faster.
Whilst this does add an awful lot of tension and atmosphere to the game, it gets old and frustrating VERY quickly. The fact you seem to be the most ill prepared special forces team doesn't seem to help either - your demolition expert only carries 2 mine that are more likely to blow him up than any enemy, and the HUD is absurd (this must be the only game where the special forces team insist on fighting on a space station wearing a narrow letter box for glasses, a motion sensor has 90 seconds of batteries and cannot be replaced and is trained to lie on the ground instead of walking backwards).
There are one or 2 nice things about the game - the cartridge based format of the Megadrive surprising benefits the atmosphere in one way, as there are not interrupting loading screens, only amusingly mundane elevator music as you travel from level to level, and returning to a previous floor is seamless (and necessary, if you're looking for ammunition and health packs)
Additionally, there is a 2 player co-operative game, that requires 2 players on 2 Megadrives and 2 TVs, but I've never had the opportunity to play it. I do imagine the ability to cover each others backs greatly removes much of the frustration, but I cannot confirm this.
On the whole here, there really isn't anything that isn't done better in Duke Nukem 3d or Doom.
This is really only a must have game for the nostalgic or obsessive FPS players, and even then playing it now may drive you nuts.