This 3-scene summary provides the essential plot details of the short story "Brown Sugar":
Scene 1 introduces the main characters Rachel, Mel, and Aster who are getting ready to go to a party at Stan's house.
Scene 2 shows the girls arriving at Stan's house for the party, but it is not very lively. Aster meets Billy who offers her a cigarette.
Scene 3 jumps ahead one year later and shows the relationship that has developed between Aster and Billy centered around drug use. It ends with Billy injecting heroin into Aster for the first time and kissing her, leading to her eventual overdose and death as shown in the final scenes.