A squad of Stormtroopers try to arrest Jix (pictured above) in a bar on Coruscant. Jix is a former combat trainer for the Empire's academy and is easily able to defeat the troopers. He then sneaks into the Castle to meet Darth Vader. He is secretly working for Vader and Jix's antics with the troopers was just him showing off his skill.
Wrenga Jixton Darth Vader
Vader then orders Jix to kill Torlock, the Governor of Corulag, for considering defection to the Rebels. Vader tells him to make it look like the governor was killed by Rebels.
Jix finds Torlock's daughter, Frija, rather than the governor himself when he attacks the governor's office. He had to fight off a group of Rebels trying to kill the governor and the group of imposter Stormtroopers. Frija says the fake troopers arrested her and that her dad is missing and suspected of defecting to the Alliance.
Frija tells Jix that her dad's most trusted friend is Admiral Droon, so Jix goes to the admiral's fortress and breaks in. When he finds Torlock in a cell, he also finds that Torlock is an android. Unfortunately, Jix is then captured by Admiral Droon and his men.
Admiral Droon Torlock droid
The Admiral reveals that Jix is actually Gunnery Sergeant Wrenga Jixton, court martialed and then sentenced to Kessel, from which he's obviously escaped.
Frija then steps out, calling Droon "darling" and denying that she's ever seen Jix. We assume at this point that the "Frija" Jix met is another android.
Droon sends Jix and the Torlock droid down a pit, where they meet and kill a Dragon slug.
a dragon slug
The droid then tells Jix that Torlock himself had the droids made as decoys because he had learned that Droon and Frija was setting him up as a traitor to the Empire. The real Torlock has escaped Corulag.
Jix then attacks the admiral's fortress again and kills the real Frija while the Torlock droid goes to find his droid daughter.
Jix then brings Admiral Droon back to Vader on Coruscant, where Vader kills Droon for lying to him about Torlock's defection. We also find out that Jix saved Vader's life and then struck a bargain with him to spare his planet from Empire control if Jix became Vader's servant.