Jedi Knight: Mysteries of the Sith is an expansion for Jedi Knight: Dark Forces 2 which was a sequel to Star Wars: Dark Forces. So Mysteries of the Sith is basically the 3rd game in the (somewhat confusingly-named) series, with Jedi Knight 2: Jedi Outcast succeeding it (and further adding to the confusion).
Mysteries of the Sith is set five years after the events of Jedi Knight. Kyle Katarn is now a Jedi Master with Mara Jade as his apprentice. Initially, you play the first four missions as Katarn, then finish the game's final ten missions as Mara.
As the game begins, Katarn and Mara are inside a Rebel base that comes under heavy attack. Katarn leads the evacuation and must first find a way of destroying the laser-equipped asteroids orbiting the planet in order to clear an escape route. Afterwards, Katarn sets off to find a long lost temple of the ancient Sith, leaving Mara Jade behind. Mara is sent to strike a deal with a Hutt before being sent on a few other errands and later retreiving a stolen Jedi Holocron. Eventually, Mara Jade sets off to find Katarn and must battle through an eerie swamp and venture into the Sith catacombs. There's not really an overall story to it; more like a series of distractions until you find Katarn where he explains why he is there.
In the final few levels, you are unable to use guns so are have to use a combination of your light-sabre and your force powers against the game's toughest enemies. The spike in difficulty and change of play-style is jarring.
You move around using WASD on the keyboard, hold shift to sprint, Z and X to jump and crouch, and the space-bar to interact with switches. Number-keys switch weapons and equip items. Q/E scrolls through Force powers and F uses them. You can save at any point (Quick-Save assigned to F9) which I recommend using frequently.
At the end of each level, you can assign a couple of points to upgrade force powers. I didn't understand the interface here because the columns were labelled “Choose 1”, “Choose 2” etc., yet you could choose more than that, but certain abilities were still disabled and this wasn't clear when they would become available. There's a few offensive force abilities like Chain Lightning and Sabre Throw which the game seems to assume you will get, since the final few levels disable use of guns. Therefore, just like the previous game, it is possible to make the game impossible by your choices of upgrades.
There's a good range of enemies including Gamorrean Guards, Tusken Raiders, Grans, Noghri, Vornskrs, Storm-troopers, and various Sith. There's even a lightsabre-only fight against the massive Rancor. There's plenty of enemies to shoot, but there's also lots of platforming and hitting switches along the way.
The original game (Dark Forces) suffered from many problems in its design. In Jedi Knight, the improvements were drastic. The main advantages were driven by the vastly improved graphics; textures are much better and sharper, meaning switches are much easier to distinguish from walls, although some are still easily missed. Since it's basically an expansion pack, the graphics are similar to Jedi Knight. The FMV sequences are gone, so now the cut-scenes are just rendered using the game's 3D engine.
The design in this game hasn't changed meaning you will end up getting a bit lost, looking for a simple switch or door. There's plenty of levels were progression is made by finding a small grate to slice open and move through the ventilation shafts. These can be fairly easy to miss, and it's not obvious to newcomers that this is even possible. So throwing one in there in the first level is very frustrating.
To me, this game seems just more of the same Dark Forces 2 without improving on it. It still seems to be regarded as cult classic, but for me, it's definitely not a great game.