The game is Star Wars: The Force Unleashed, and some may declare it the last good Star Wars game that Lucas Arts made before closing down after the Disney acquisition (Though, I myself am a fan of the sequel).
The game touched upon the largely unexplored timeline set in-between Star Wars Episode III: Revenge of the Sith and Star Wars Episode IV: A New Hope.
As the game opens, the player assumed control of the Dark Lord himself, Darth Vader, as the newly established Empire attacks the Wookiee planet of Kashyyyk. The Wookiees are harboring a Jedi Knight, and Vader plans to put all remnant of the Old Republic to rest, especially the Jedi.
Vader slices, force chokes, and obliterates Wookiee resistance until the Jedi, Kento Marek reveals himself. The Sith Lord makes quick work of Marek, shortly discovering that the Jedi has a son... a son very powerful in the force.
Darth Vader has taken it upon himself to train Kento Marek's son, Galen, in the dark side of the force. Years later, Galen, now codenamed Starkiller, has become Vader's secret apprentice and an assassin that specifically targets Jedi. Starkiller, accompanied by his holodroid, Proxy and his pilot, Juno Eclipse, is sent on a mission to assassinate Jedi Rahm Kota, who has attacked an Imperial ship yard with the help of his militia from the Clone Wars.
As the player assumes control of Starkiller it feels as though anything is possible. The Force Unleashed truly does make the player feel as though the force is the ultimate weapon.
Anyway, Starkiller manages to defeat Kota by throwing him out of the falling shipyard and returns to Vader, where he is tasked with with hunting down Kazdan Paratus, a Jedi driven insane by hiding in isolation from the Empire on the Junkyard planet of Raxus Prime. And shortly after killing Paratus, Vader tasks Starkiller with killing a true Jedi Master, Shaak Ti, who is hiding on the planet Felucia with her padawan Mariss Brood.
Starkiller engages in a fierce battle with Shaak Ti and emerges victorious. However, upon returning to Vader, Starkiller is betrayed. Emperor Palpatine, also known as Darth Sidious, has discovered that Vader is keeping a secret apprentice. Vader plunges his lightsaber through Starkiller's body, then launches him into space. After all, Sidious never permitted Vader an apprentice, and when it comes to Sith there can only be two, a master and an apprentice. Vader wouldn't want Sisious to get the idea that he planned to overthrow him, now would he?
Vader 's medical droids find and restore Starkiller and Vader tells him that it was all a ruse to throw the Emperor off of his trail. Vader gives Starkiller the mission of charading as a Jedi and assembling all the would-be rebels against the Empire so that Vader can put an end to them. Starkiller escapes his captivity, rescues his pilot Juno, who the Empire were going to execute, and plots a course for Naar Shadaa where he senses the presence of... Rahm Kota?
Kota did indeed survive the duel with Starkiller, though he is now blind. Starkiller is trained by Kota in the ways of the Jedi, and what has begun as a charade may become something more.
Starkiller begins to discover a new path as he goes to Kashyyyk and meets with his father's force spirit... he discovers the path of the Jedi and soon becomes the foundation for the Rebellion.
This is a phenomenal story set in the Star Wars universe, with an alternate ending that leads to a universe where the original trilogy had a very different outcome...