Geek Trivia: What other sci-fi franchise unofficially 'shares' a universe with Hitchhiker's Guide to the Galaxy?

Douglas Adams wrote several early drafts of the Hitchhiker's Guide movie script -- and made a special effort to secretly tie the movie universe into another well-known science-fiction franchise.

According to The Hitchhiker's Guide to the Galaxy movie, it's entirely possible that a certain Gallifreyan Time Lord could someday find himself at the Restaurant at the End of the Universe, as Hitchhiker's shares a universe with Doctor Who.

Warning, very mild spoilers ahead.

When Arthur Dent is touring the Magrathean planet assembly line in the Hitchhiker's Guide film, the prison planet Shada is visible on the factory floor. "Shada" was the title of a 1979 Doctor Who serial that was never aired due to a BBC strike. The series was written by Douglas Adams. The BBC eventually released the incomplete Shada storyline on video, with the "ending" performed via narration by Tom Baker, who portrayed the Fourth Doctor.

(Yes, Douglas Adams wrote for Doctor Who. He also wrote for Monty Python and played guitar with Pink Floyd. It's entirely possible his untimely demise was due to an overdose of self-generated awesome.)

To double down on the time-space crossover, characters and settings from another Adams-cowritten Doctor Who serial, "City of Death," appear in Adams's other famous comedy novel series, Dirk Gently's Holistic Detective Agency. It seems wherever Douglas Adams's fiction went, the Doctor was sure to secretly follow.

That's not just some Time Lord-entangling text twisting; it's a fandom-friendly flash of Geek Trivia.