Star Wars The Force Unleashed is a game that fills in the unexplored bits and pieces of the Star Wars Timeline. What we seen in the movies is that Obi-Wan and Yoda survive Order 66. But three other Jedi survived: Rahm Kota. Kota survived because he chose not to use Clone Troopers in battle. Instead he used ordinary humans. Another Jedi that survived is Shaak Ti. Shaak Ti is a member of the Jedi Council. She went to planet Felucia in hopes that she can assemble a new army of Jedi Knights to take down the Emperor. And the last Jedi is Maris Brood, a Padawan. She is Shaak Ti's second-is-command and Padawan apprentice.
In The Force Unleashed you play as Galen Marek. Galen was Darth Vader's "secret" apprentice, sent on top secret missions to kill any surviving Jedi. After killing Shaak Ti, Marek is troubled with her last words: "The Sith betray each other". And when he returned to the Executor (Vader's flagship. Also the biggest Star Destroyer in the Imperial Fleet), he meets Emperor Palpatine (a. k. a. Darth Sidious, Darth Vader's master). Galen is shocked to see that Vader has betrayed him (he literally back stabbed Marek.) after a painful defeat, Vader thrashes Marek all around the bridge before ending the fight by using the Force to throw Galen Marek into space. Six months later, Galen reawakens in Vader's private science lab, which nobody except Vader himself and medical droids are allowed to access. Turns out that Vader has secretly recovered Marek's body and has been rebuilding him. Vader also tells Galen that he "saved" him from death (Marek was fooled because he had no memory of the fight he had with Vader). Galen is now charged with the task to form the Rebel Alliance. After doing that (I'm not going to tell everything that happened while forming the Alliance, then we would be here forever) Galen visits a house on planet Kashyyyk. After a force ghost visit, Galen discovers that the house was where he lived before becoming a Sith, learns his real name (this is where everybody calls you Galen Marek as when you were slaughtering Jedi you were called Agent Starkiller and after that Vader fight you were called Subject Zeta), and get a new water blue colored lightsaber. Now that Galen formed the Rebel Alliance, they need a target to prove that the Empire is weak. So they attack an Imperial ship yard that has a TIE Starfighter factory on the planet below. After taking the Factory out, Galen does the most AWESOME thing ever that you can do with the Force. He rips an Imperial Star Destroyer out of orbit and slams it into the ground, making the whole thing EXPLODE!!!!!!!!!!!! After that, Princess Leia (and some other dudes) fall for a carefully planned trap. You see, Vader told Galen to form the Rebel Alliance to gather all the Empire's enemies. Then, Vader told Galen to attack the ship yards to lure the Alliance Fleet away from headquarters. And finally, while the Fleet was too busy attacking the ship yards, the Imperial Fleet would attack headquarters and take the leaders prisoner where they will be executed on Death Star 1. After rescuing the leaders, you can choose one of two endings: If you spare Vader's life and engage and defeat Sidious, where Juno Eclipse (A Friend of Galen Marek.) prevents Galen from killing the Emperor. But the Sith Lord kills Galen Marek by shooting lightning, but retreats because he's too weak to continue fighting. But if you engage and defeat Darth Vader, Galen Marek killed Darth Vader and becomes Palpatine's new apprentice. And in all the scenes in the movies A New Hope and The Empire Strikes Back featuring Darth Vader are replaced with scenes exactly the same as the original but with Vader replaced with Galen Marek.
logo of series. silhouette is Galen Marek
Well since I gave away the story of the first game, I won't give away spoilers for the second game. Instead, a short description.
In The Force Unleashed 2, you play as a clone of Galen Marek (as the original died at the end of the first game). The clone is....................................................unstable. He's got more Force power than the original, he uses two lightsabers at a time and has faint memories of the original Galen Marek (all of these are side effects of the instability). So he sets out on a quest to discover the legacy of the original Galen Marek, and discover his past.