The game begins with the cut scene of the events of the last prequel movie, Revenge of the Sith. The Jedi are being wiped out by

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both the clone troops and that of the newly acquired Anakin Skywlker. The prologue is a bit like a tutorial, where you take control of the new Sith Lord, Darth Vader on the planet of Kashyyyk. This bit goes into how to use your Force powers, what the controls are, how to move, etc.
The point of this mission is to find the last Jedi on the planet, but you will be facing Wookie forces that will try and stop you. Players who would normally play light sided characters will need to be strong to test out their powers on Chewbacca's kin, but it will give you an idea on how to use your powers for levitation, Force Push, and of course the fan favorite of Force Lightning.
The boss Jedi at the end of the Prologue is the Rogue Jedi. There will be a cut scene in which he will try and stop you from reaching the huts behind him. This fight can be tricky, as you are still learning the different sets of power and you will need to make sure to use either Force Push or Repulse when the Jedi begins to fling out his lightsaber.
You need to quickly bring his health down, so start out by using Saber Throw, and then follow with a quick combo and his health should be taken down a notch. When this Jedi master starts to get weak, he will pull out his biggest attack, which will be pulling a passing TIE fighter right out of the sky and dropping it on top of Vader's head. Pull out just before this attack and keep moving around and you should miss getting hit by most of the explosion, and then finish this master Jedi.
When you have brought down the Jedi's health enough, this will trigger a cut scene in which Vader tortures the man for information. Before the Sith Lord can continue, a small boy approaches and tries to save the man. Vader is impressed by the boy's talent and recruits him as his apprentice.