Set between the events of Revenge of the Sith and A New Hope, the game begins with Darth Vader being sent by Emperor Palpatine, his Sith Master and ruler of the Galactic Empire, to hunt down a Jedi Knight and survivor of the Great Jedi Purge named Kento Marek on the Wookiee homeworld of Kashyyyk. Throughout the game’s first level, the player controls Vader as he slaughters Wookiees during the Imperial invasion of Kashyyyk, eventually finding and defeating Marek. As he prepares to kill him, Vader discovers Marek’s son Galen who is strong with the Force. Upon murdering Marek, Vader takes Galen and raises him to become his apprentice, unbeknownst to the Emperor.

Years later upon reaching adulthood, Galen (referred to as «Starkiller» by Vader) completes his training and is sent by his master to kill a select group of surviving Jedi Masters across the galaxy as a final test to prepare him for Vader’s ultimate goal: to assassinate the Emperor so that Vader and Starkiller could rule the Empire. Starkiller travels between missions aboard his personal ship the Rogue Shadow developing a close relationship with his crew, consisting of lightsaber training droid PROXY and Imperial pilot Juno Eclipse. Starkiller’s targets include Rahm Kota, a former general that fought in the Clone Wars and led his own militia, who attacks a TIE fighter shipyard to lure out Vader; the insane and isolated Kazdan Paratus, who built a replica of the Jedi Temple on Raxus Prime; and Shaak Ti, who controls the wildlife of Felucia through the Force, and warns Starkiller of Vader’s inevitable betrayal before committing suicide. After all the Jedi are eliminated, Vader reveals Starkiller to the Emperor, only to swiftly betray and kill him to prove his loyalty to his master. Unbeknowst to the Emperor however, Vader later has Starkiller revived, believing that he still has his uses.

Once Starkiller recovers, Vader sends him to foster a rebellion among those who resist the Empire, explaining that this is all part of a master plan to destabilize the Empire and create the conditions necessary to depose the Emperor. Against Vader’s orders to cut his ties to his past, Starkiller rescues Juno, who was captured and branded a traitor to the Empire, and escapes with her and PROXY via the Rogue Shadow. Looking for allies to recruit into the rebellion, Starkiller first approaches Kota on Nar Shaadda, who survived their last fight, but was blinded and reduced to alcoholism. Upon escaping from Imperial forces looking to arrest them, Starkiller and Kota travel to Kashyyyk, now under Imperial occupation, to recruit Princess Leia Organa. After experiencing a vision of his father’s spirit in his old family home who tells him it’s not too late to redeem himself, Starkiller proceeds to liberate the enslaved Wookiees, earning Leia’s gratitude, who agrees to join the rebellion. Starkiller next heads to Felucia to rescue Leia’s adoptive father, Senator Bail Organa, from Shaak Ti’s former apprentice Maris Brood, who has succumbed to the dark side after her master’s death alongside the rest of Felucia’s inhabitants, using them to wage war on Imperial forces trying to occupy the planet. After defeating Brood, Starkiller spares her life and rescues Organa.

To convince more senators to join the rebellion, Vader suggests that Starikiller attack a Star Destroyer facility on Raxus Prime to show that the Empire is vulnerable. During his mission, Starkiller is attacked by PROXY, who attempts to kill him to fulfill his programming, but Starkiller defeats him. He then destroys the facility, and pulls a Star Destroyer out of the sky using only the Force. Following this, Bail Organa meets with fellow Senators Mon Mothma and Garm Bel Iblis on Corellia to formally organize a rebellion, only for Vader to come and arrest them and Kota. After defeating Starkiller, Vader explains that he was merely a tool to lure out and destroy the Emperor’s enemies and that his plans to overthrow the Emperor never included him; however, Starkiller survives thanks to PROXY sacrificing himself to distract Vader.

Using the Force to ascertain that the senators and Kota were taken to the Death Star, Starkiller goes there with Juno, with whom he shares a kiss before parting ways. Starkiller fights his way to the Emperor’s throne room, whereupon Vader arrives to confront him. After defeating his former master, the Emperor tries to goad Starkiller into killing Vader and replacing him as his new apprentice, but Kota attempts to attack the Emperor from behind and is subdued with Force lighting. At this point, Starkiller must choose between saving Kota, or finishing off Vader.