Jedi Knight: Mysteries of the Sith (commonly abbreviated MotS) is a game created by LucasArts in 1998. It was created as an expansion sequal to Jedi Knight: Dark Forces 2, featuring its own new story.
The game mostly revolves around the character Mara Jade, a former Imperial assassin and trainee of the Emporer-turned jedi. Luke Skywalker brought her to be trained by Kyle Katarn in the ways of the jedi.
The game begins on Altyr 5 with Kyle and Mara practicing lightsaber dueling. Imperials suddenly attack the New Republic base out of nowhere. Kyle and Mara are informed that they are attacking using a starship disquised as an asteroid. Kyle heads for an Imperial shuttle to destroy the asteroid. There, he redirects its cannons on itself and destroys the cooling system, causing a reactor failure, and therefore, destroying the base entirely.
When Kyle gets back, he tells Mara that he must leave again to investigate a planet called Dromund Kaas. Imperial activity has been detected near the planet. While gone, Mara is given the task of acquiring equipment from Ka'Pa the hutt. At his palace, the front door quard denies Mara access, claiming that Ka'Pa does not wish to see her. After finding another way in, Mara explores the palace, at some points in disguise.
Mara then enters Ka'Pa's main chamber by blowing up the roof. Ka'Pa makes a somewhat irrefutable deal with Mara to steal Takara's global communications transponder, in return for the equipment promised. Mara then makes her way to the Katraasii spaceport to hunt down Abron Mar, a lieutenant in Takara's criminal organization. When she finally closes in on him, his goons ambush her, stunning her before she realizes it was a trap.
Mara then wakes up in a prison cell with all her weapons missing, including her lightsaber. She then escapes the cell and travels through a pipe system, ending up in a Rancor cave. She finds her lightsaber within the cave and kills the Rancor, after which she makes her way through the rest of Takara's stronghold, before retrieving the GCT device and escaping.
Later, Mara is escorting a New Republic ship carrying a much sought-after Jedi Holocron, only for the ship to be attacked by pirates. Mara defeats many of the pirates, but fails in protecting the Holocron. It, and much of the other cargo, is stolen by the pirates. Mara, thinking quickly, hides inside a crate which is then taken by the pirates, allowing Mara to stow away on their ship.
She exits the crate, and learns that the Holocron is being taken to the base of a private collector known as Kaerobani, a notorious criminal who collects rare artefacts out of selfish greed. Mara manages to steal one of the pirates' transport ships and flies to Kaerobani's base by herself.
She manages to reclaim the Holocron, but has to fight her way out of Kaerobani's base, before bringing it back to the New Republic. She learns that Kyle has yet to get back in contact, and she fears what he may have found on Dromund Kaas
Mara travels to the swampy planet of darkness, and eventually makes her way to a Sith Temple Kyle said he was attempting to find. She enters and discovers many dark temptations, before confronting Kyle in the core of the temple, bringing him back from the very edge of no-return before it was too late.
Despite being dubbed as an "expansion", it is a standalone game, and does not require the original Jedi Knight to play on its own. It does require Dark Forces 2 if it is to be installed from the CD, as it asks for the second disk from Jedi Knight for verification. It is possible to bypass this by manually installing the game from the disk's files. This method is usually required on newer operating systems (notably 64-bit ones).