The titular film is William Shakespeare's Romeo + Juliet. This song was written after the band were screened the last half hour of the movie. Thom Yorke's original idea was to use lines from the play as lyrics, but apparently that didn't work out.
The most direct inspiration for the song was the moment in the film in which Juliet holds the Colt .45 to her head. But Thom was also thinking about the 1968 version of the film:
"I saw the Zeffirelli version when I was 13 and I cried my eyes out, because I couldn't understand why, the morning after they shagged, they didn't just run away. The song is written for two people who should run away before all the bad stuff starts. A personal song."
source: green plastic radiohead