There are two points, one at the beginning and one at the end of the film, that make up the theory.
Then, at the end, we see Danny and Sandy driving away and the car they’re in begins to fly up into the sky.
If you’re somehow not familiar with this classic theory, it supposes that Sandy, the female lead of Grease played by Olivia Newton-John, is actually dying throughout the entirety of Grease and that the film is actually a fantasy created in her mind as she dies.
The song “Summer Nights” sung early in the film tells of Danny (Travolta) and Sandy meeting during the previous summer, and Danny sings a line claiming that Sandy nearly drown but he saved her life.