Lucas wrote the story to Empire Strikes Back and should have had first billing on Screenplay. Lawrence Kasdan came in from Raiders of the lost ark and did work. Kasdan and Lucas would work again on the screenplay for Return of the Jedi. Nothing Leigh Brackett did on Empire was used.
Wikipedia is the source I'm using in the text below. Read the Making of The Empire Strikes Back(enhanced edition) for a more in depth reading of Lucas involvement with empire. Chapter two in a section called Misfire. It's a book by Rinzler, J.W.
Lucas hired science fiction author
Leigh Brackett to write
Star Wars II with him. They held story conferences and, by late November 1977, Lucas had produced a handwritten treatment called
The Empire Strikes Back. The treatment is similar to the final film, except that Darth Vader does not reveal he is Luke's father. In the first draft that Brackett would write from this, Luke's father appears as a ghost to instruct Luke.
[18]
Brackett finished her first draft in early 1978; Lucas has said he was disappointed with it, but before he could discuss it with her, she died of cancer.
[19] With no writer available, Lucas had to write his next draft himself. It was this draft in which Lucas first made use of the "Episode" numbering for the films;
Empire Strikes Back was listed as
Episode II.
[20] As Michael Kaminski argues in
The Secret History of Star Wars, the disappointment with the first draft probably made Lucas consider different directions in which to take the story.
[21] He made use of a new plot twist: Darth Vader claims to be Luke's father. According to Lucas, he found this draft enjoyable to write, as opposed to the yearlong struggles writing the first film, and quickly wrote two more drafts,
[22] both in April 1978. He also took the script to a darker extreme by having
Han Solo imprisoned in
carbonite and left in
limbo.
[23]
This new story point of Darth Vader being Luke's father had drastic effects on the series. Michael Kaminski argues in his book that it is unlikely that the plot point had ever seriously been considered or even conceived of before 1978, and that the first film was clearly operating under an alternate storyline where Vader was separate from Luke's father;
[24] there is not a single reference to this plot point before 1978. After writing the second and third drafts of
Empire Strikes Back in which the point was introduced, Lucas reviewed the new backstory he had created: Anakin Skywalker was Ben Kenobi's brilliant student and had a child named Luke, but was swayed to the dark side by Emperor
Palpatine (who became a
Sith and not simply a politician). Anakin battled Ben Kenobi on the site of a volcano and was wounded, but then resurrected as Darth Vader. Meanwhile Kenobi hid Luke on
Tatooine while the Republic became the Empire and Vader systematically hunted down and killed the Jedi.
[25]
With this new backstory in place, Lucas decided that the series would be a trilogy, changing
Empire Strikes Back from
Episode II to
Episode V in the next draft.
[22] Lawrence Kasdan, who had just completed writing
Raiders of the Lost Ark, was then hired to write the next drafts, and was given additional input from director
Irvin Kershner. Kasdan, Kershner, and producer
Gary Kurtz saw the film as a more serious and adult film, which was helped by the new, darker storyline, and developed the series from the light adventure roots of the first film.
[26]