After Star Wars: A New Hope and Star Wars: The Empire Strikes Back, it was clear that George Lucas had something hot in his hands. At this point, Lucas was under some pressure, and it would've been easy to produce a followup that was a disaster and let everyone down. Star Wars: Return of the Jedi shows Lucas' ingenuity: he combines the best elements of the previous two episodes and produces a movie that is not only technologically advanced, in terms of special effects, but ties the loose ends together in a consistent whole.
The plot is similar to Star Wars: A New Hope. The Evil Empire has built a new killing machine, much more powerful than the Death Star (even though it has the same weak points as far as blowing it up is concerned). The Rebel Alliance must destroy it before it destroys them.
Luke Skywalker (Mark Hamill), along with Princess Leia (Carrie Fisher), Chewbacca (Peter Mayhew), Lando Calrissian (Billy Dee Williams), C3PO (Anthony Daniels), and R2D2 (Kenny Baker) tie up the first loose thread by rescuing Han Solo (Harrison Ford) from the clutches of the evil Jabba the Hutt (Toby Philpott). Then Luke returns to the Dagobah system to finish his training (thread number two) with Yoda (Frank Oz) and makes it there just in time. The nine-hundred-year-old Jedi Master dies and joins Obi-Wan Kenobi in the great beyond. Luke learns from Yoda that he has a twin sister and must confront Darth Vader before he can become a proper Jedi Knight (and that's two more threads tied up for you).
Meanwhile, the Rebel Alliance begins planning their attack on the Empire's weapon. Luke and his friends are responsible for destroying the shield protecting the new weapon so Lando and the Alliance crew can destroy the revamped Death Star. As they approach Endor, the moon containing the shield controls, Darth Vader (David Proust, voice of James Earl Jones) and Luke sense each other's presence. Luke then confronts Darth Vader and tries to convert him to the good side.
Up till now, the most fearsome villain in the Star Wars movies was Darth Vader. But Lucas manages to create an even more impressive one, just as he begins to change Darth Vader into a good guy. The new villain is Darth Vader's master, the evil Emperor (Ian McDiarmid), who looks feeble and old but possess extraordinary Jedi powers (presumably from his alliance with the dark side). When the Emperor realises he cannot convert Luke to the dark side, he begins to slowly and painfully kill Luke. Darth Vader, seeing Luke lying helplessly on the floor while the Emperor exhibits pure evil, feels a spark of compassion and kills the Emperor (a little too easily, but it's better than having a sustained fight). In doing so, he finally redeems himself and joins Obi-Wan and Yoda.