Sometime between Darth Vader being a pussy and becoming a badass, he was a massive jerk. For some reason, this entitled him to his very own sith bitch, which is where The Force Unleashed comes into play.
Well, yes and no. Yes, because it's George Lucas and no, because it's actually quite fun to play once you get past the annoying story. Imagine the Star Wars series is a puzzle. For years, it was half done. Then, BAM, old Lucas came out of retirement and made the prequel trilogy. It may not have been perfect, but the puzzle was finally finished. Then, out of nowhere, the people responsible for the franchise said "Hey, that puzzle's not finished!". You look at them, bemused. "Of course it is, look, I--" but alas, there's suddenly a big gap in the middle. They then charge you $50 for the extra piece, which isn't exactly neccesary to the rest of the puzzle but without it, the whole thing looks silly. So you buy the piece, content that the puzzle's done. But, oh no, they're not done with you yet. It happens again. This time, the piece is even more insignificant.
Granted, if the missing piece was Vaders head, we'd probably hand over the cash
Basically, Vaders purging the Jedi survivors who didn't get shot by clones in Episode 3. During one such mission, he senses a strong presence in the force, which turns out to be the son of the Jedi he's hunting. He takes the boy in and trains him as his apprentice, with the soul purpose of overthrowing The Emperor in the future.
"The force may be strong in you, but your advertising abilities are atrocious. You're Fired!"
The first Jedi on the hit list is one Rahm Kota, a old fella who is (predictably) the Obi-Wan of this story. Starkiller doesn't kill him, but he does manage to blind him with his own lightsaber. After a number of assassinations, Starkiller heads back to Vaders Star Destroyer, only to be betrayed by his master in an amazing display of treachery and douchebaggery. Apparently, The Emperor found out about Starkiller and suspected that Vader was going to betray the Empire. Starkiller's thrown out of the window into space, only to be rescued later by Vader, claiming he needs to start a rebellion in order to distract The Emperor.
"What? I don't see it? What am I mean to be looking at here?"
After finding Kota, who's been trying to drink his troubles away (like the best of us), Starkiller goes on a series of explorative missions to round up the usual suspects (Bail Organa, Princess Leia, etc) and form a rebel alliance. Their first meeting doesn't go very well, due to an imperial fleet led by Vader crashing the party. Turns out, he's been using Starkiller to round up the Empires enemies and bring them into the open (Who didn't see this coming?). After kicking Starkiller off a cliff, Vader takes the rebels back to the Emperor. Unhappy that his adoptive pop kicked his ass, Starkiller goes to the Death Star (estimated completion time, 2-3 years). to hand out some whoopass. He manages to save the rebels, but dies in the process, therefore becoming a symbol for the galaxy to rally behind.
There is a love interest in there somewhere, but she's so dull, she may as well be made of paper.