The Hitchhiker's Guide to the Galaxy (in-universe)

The Hitchhiker's Guide to the Galaxy was a play that Romana II once chose to attend over A Midsummer Night's Dream. She called it a comparatively "much more useful play, and much funnier too". (PROSE: A Midsummer's Nightmare)

It was also an Earth radio programme written in 1976 by Douglas Adams and John Lloyd. (WC: Tom Baker stars in John Lloyd's lost Doctor Who adventure, The Doomsday Contract)

Behind the scenes
Lines and characters from Hitchhiker's have appeared in Doctor Who.
 * The Seventh Doctor once asked, rhetorically, who had said that "Earthmen rarely invite their ancestors to dinner", which came from the series. (TV: Ghost Light) The Master asks a near identical question, albeit with different wording when attempting to recover his TARDIS alongside River Song on Cheska Minor. (AUDIO: Animal Instinct)
 * The number 42, which in the Hitchhiker's series was the answer to the question of life, the universe, and everything, was one of the numbers the Tenth Doctor guessed when trying to find out the security protocol for the Host. (TV: Voyage of the Damned)
 * When the Doctor was pointing out species to Clara, one of the species were the Hooloovoo, a species name in Hitchhiker's. (TV: The Rings of Akhaten)
 * When the Fourth Doctor spoke to the Hornets, he mentioned the Vogons, another species name in Hitchhiker's. (AUDIO: A Sting in the Tale)
 * Lakksis uses the phrase "Life, don't talk to me about life", often used by Marvin in the series. (PROSE: Wish You Were Here)
 * The Tenth Doctor, while on the Sycorax spaceship in 2006, mentioned Arthur Dent and said that he was a "nice man". (TV: The Christmas Invasion)

Real world connections
Hitchhiker's creator Douglas Adams wrote a number of Doctor Who serials and served as its script editor for Season 17. Consequently, lines from Hitchhiker's Guide (including the famous instruction "DON'T PANIC") found their way into TV: The Pirate Planet, while Hitchhiker's character Oolon Coluphid and Betelgeuse gets a mention in TV: Destiny of the Daleks, which Adams script-edited (the Doctor is seen reading one of Coluphid's books, Origins of the Universe). Another possible reference is the name of the planet Bandraginus V whose name closely resembles Santraginus V, a planet from Hitchhiker's. (TV:The Pirate Planet) The storyline of Adams', the third book in the " Hitchhiker's series, was based on a rejected Doctor Who script called Doctor Who and the Krikkitmen. (This was later novelised by James Goss.) Adams later reused the character of Professor Chronotis in his novel.

Because Paddy Kingsland did special sounds for both the Hitchhiker's Guide radio programme, and for The Sun Makers, there are sounds which are common to both productions. Notably, the sound of the Fourth Doctor fiddling with a combination lock in part 4 is the same as the sound heard in Hitchhiker's whenever the Guide itself was being consulted. (DCOM: The Sun Makers)

Just prior to becoming the Fifth Doctor, Peter Davison made a cameo appearance in the BBC's 1981 adaptation of the first book as the "Dish of the Day". His wife, Sandra Dickinson, played Trillian in the miniseries.

After Douglas Adams' death, Eoin Colfer was given permission by Adams' widow to write an additional novel in the series. This became And Another Thing.... Colfer later wrote PROSE: A Big Hand for the Doctor for the Puffin E-books range.

Metafictional parallels and references

 * The plot of TV: Voyage of the Damned is similar (but not identical) to that of, a video game authored by Adams which was published in 1998. Both feature a large luxury spaceship/cruiseliner named "Titanic" which goes out of control and whose computers must be manipulated to fix the ship. The video game was based on a brief mention of the ship in the first Guide book, which was unable to send out its first and only message - an S.O.S. - during its launch before suffering a "total existence failure".
 * TV: 42 shares its title with the, as revealed in Adams' . As with the spaceship stolen by Arthur Dent and friends in , the ship in 42 is on its way to crashing into a star, leaving its passengers with no escape.
 * The reference to Oolon Coluphid has raised speculation as to whether the Doctor Who universe and that of the Hitchhiker's Guide are one and the same. Another matter blurring the lines between the Hitchhiker's universe and the Whoniverse is the Tenth Doctor's early reference to Arthur Dent being a "nice man" who saved the universe in "his jim-jams"; it's left ambiguous as to whether he's referring to Dent as a fictional character or a real person. (TV: The Christmas Invasion) Also, it is mentioned in Doctor Who (though not proven or brought up further in any later episode) that humans may have evolved from an extraterrestrial species. (TV: Image of the Fendahl) This might have some relation to the idea of humans being descended from Golgafrinchans in Hitchhiker's Guide.
 * Perhaps by a coincidence, the scene in the third book of the quintilogy (later adapted into a radio script of the same name) Life, the Universe, and Everything, in which Arthur Dent and Ford Prefect, materialise in Lord's Cricket Ground, is similar to a scene in The Daleks' Master Plan in which the First Doctor materialised his TARDIS at the Oval.
 * On the second DVD disk of The Hitchhiker's Guide to the Galaxy TV series, a TARDIS in policebox form can be seen tracking the ship upon which Arthur is getting a lift, during the opening credits of the 'The Making of...' documentary. The ship is the Liberator from 'Blakes 7', a BBC TV programme made between 1978 - 1981 and sharing many cast and crew with 'Doctor Who'.
 * On the website "Who is Doctor Who", Arthur Dent says that the Ninth Doctor lay down in front of a bulldozer in front of his house, calling him 'rather odd'.
 * One of the spaceships in AUDIO: Max Warp is described as a "Lazlar Lyricon custom job". This is how one of the spaceships in the Milliways car park in episode 5 of HHGG was described.
 * As highlighted by the BBC official Doctor Who website, there is a copy of The Hitchhiker's Guide to the Galaxy in the Library as seen in Silence in the Library.
 * When Adams' unfinished television story Shada was remade by Big Finish Productions, writer/producer Gary Russell introduced a few references to the Hitchhiker's series as a tribute. These included making the car which Skagra steals into a (and its owner the treasurer of the Ford Prefect Appreciation Society), and, in the webcast version, showing a  machine in the background of the lab and silhouettes of  and  as two of the prisoners in Shada. (WC: Shada, AUDIO: Shada)
 * Similarly, when author Gareth Roberts novelised Shada in 2012, he added an implication that Professor Chronotis had replaced The Worshipful and Ancient Law of Gallifrey with a copy of The Hitchhiker's Guide to the Galaxy. Roberts had Chronotis launch into a truncated speech which appeared to extoll the virtues of the Guide. "...an Earth classic by one of the greatest writers in that planet's history," Roberts had Chronotis say, "... Terribly funny, terribly thoughtful, wish I could remember the name of it, something about thumbing a lift, and there were towels in it ... oh yes, of course, it's called The Hitch—" before being interrupted. (PROSE: Shada)
 * Two references in The Pirate Planet (as well as its novelisation), madranite one-five (a mineral) and Qualactin (a planet), are both mentioned in The Hitchhiker's Guide to the Galaxy. Madranite is a mineral in the Mining Belts of Orion Beta and Qualactin Hypermint extract is an ingredient in the Pan Galactic Gargle Blaster.