The Hitchhiker's Guide to the Galaxy (radio series)

A Radio Drama on BBC Radio 4 which later became the basis for various spin-offs. It (technically) ran for twenty-six years and as many episodes.

It started with only two seasons in 1978-1980. It was later mixed-and-matched into the first two novels. It was later revived in 2003 and spawned three more seasons based on the last three books by Douglas Adams.

The cast was fairly consistent across the board, with only Peter Jones and Richard Vernon being replaced in the last three seasons due to existence failure. Even death didn't stop Douglas Adams from putting in an appearance in Season 3, despite the fact that he wasn't reprising a previous character.

Hipgnosis did the cover art for the British releases of the soundtrack albums.

The original two seasons contain examples of:
""It's a sort of threat, you see...""
 * Acting for Two: Bill Wallis plays both Prosser (the man in charge of demolishing Arthur's house) and Jeltz (the alien in charge of demolishing Arthur's planet).
 * Deadly Euphemism: Played with.
 * Slartibartfast threatens Arthur that, unless Arthur comes with him promptly, he will be late -- as in "the late Dent Arthur Dent".

"Zaphod: Oh, god. [rapidly] Ford, this is Trillian, Hi. Trillian, this is my semi-cousin Ford who shares three of the same mothers as me. Hi. [...] Zaphod Beeblebrox, this is a very large drink. Hi."
 * Hig Hurtenflurst explains his use of "revoked" to Arthur by spelling it out as "k-i-l-l-e-d". A subsequent episode reveals that this is part of a larger legal wrangle where (for various reasons) the representatives of a cloning agency were trying to get murder redefined in law. They'd managed to have the word legally changed, but not the spelling.
 * Idiosyncratic Episode Naming: Fit the First, Fit the Second, etc. This is a reference to Lewis Carroll's The Hunting of the Snark.
 * Idiosyncratic Season Naming: Primary Phase, Secondary Phase, etc.
 * I Need a Freaking Drink: When it's established that (due to the effects of the Infinite Improbability Drive) Ford and Trillian are the only two members of the core cast who haven't coincidentally met before:

"The Haggunenons of Vicissitus Three have the most impatient chromosomes of any life form in the Galaxy. Whereas most races are content to evolve slowly and carefully over thousands of generations, discarding a prehensile toe here, nervously hazarding another nostril there, the Haggunenons would do for Charles Darwin what a squadron of Arcturan stunt apples would have done for Sir Isaac Newton. Their genetic structure, based on the quadruple sterated octohelix, is so chronically unstable that, far from passing their basic shape onto their children, they will quite frequently evolve several times over lunch."
 * Large Ham: Zaphod, played by Mark Wing-Davey, was written as such specifically because he was played by Mark Wing-Davey.
 * Message in A Bottle: Arthur and Ford are stranded on prehistoric Earth, and attempt to attract the attention of a passing spaceship by waving a towel at it. A volcano then erupts, covering the towel with lava. When the Earth is blown up six million years later, the now-fossilized towel gets launched into space and found by Zaphod Beeblebrox in the spaceship Heart Of Gold, who travels back in time and rescues them. (Things like this tend to happen whenever you use the Heart Of Gold's "Infinite Improbability" drive.)
 * Relax-O-Vision: During the approach to Magrathea where, supposedly in order to help combat rising stress levels in the galaxy, it was carefully explained to the audience that no one was going to get killed in the ensuing confusion -- although one unidentified person would be bruised on the arm.
 * Retronym: It wasn't until the radio series were released on tape that the seasons began being referred to as Phases. (This only applies to the first two seasons, the rest were labeled Phases right out of the gate.)
 * Second Episode Introduction: Zaphod, Trillian, and Marvin.
 * Shaped Like Itself: Zaphod and Ford find themselves in a cave made out of marble, very slippery marble. Zaphod tries to compare it to the slipperiest thing Ford can think of. Unfortunately, the slipperiest thing Ford can think of is the marble, leading to the statement "This marble is as slippery as this marble."
 * Unstable Genetic Code:

"Arthur Dent: What happens if I press this button?
 * What Does This Button Do?: Lampshaded.

Ford Prefect: I wouldn't--

Arthur Dent: Oh.

Ford Prefect: What happened?

Arthur Dent: A sign lit up, saying "Please do not press this button again.""

The three revived seasons contain examples of:

 * And There Was Much Rejoicing: The final fate of Wowbagger.
 * Back for The Finale: The ending sequence of the final episode of the Quintessential Phase (and the final episode of the radio series altogether) has the return of Fenchurch and Marvin, as well as Max Quordlepleen, the Great Prophet Zarquon, Wowbagger, Lintilla, Prosser, and a few other bit players.
 * Casting Gag: Geoffrey Perkins, Douglas Adams's boss at the BBC, plays Arthur Dent's boss at the BBC.
 * Content Warnings:
 * One episode of the Tertiary Phase finishes with the warning: "The preceding program contains scenes of extreme violence which may be disturbing to some viewers. Time travellers of a nervous disposition may wish to consider listening to something else for the previous half-hour."
 * Played straight when the episode featuring the destruction of the Guide offices was broadcast on Radio 4 shortly after 9/11.
 * Hey, It's That Voice!: Stephen Fry is Arthur's friend Murray Bost Henson.
 * Remake Cameo: Inverted--cast members of the remake play roles in the continuation of the original series:
 * The Quintessential Phase series had Sandra Dickinson, Trillian in the TV version, voice Tricia McMillan (Trillian's alternate universe counterpart).
 * David Dixon, the TV Ford Prefect, also had a cameo in the second episode of the Quandary Phase, getting pissed off at Arthur for trying to donate to save the dolphins when he should know they've all vanished. (This case is lampshaded, as Arthur--still played by Simon Jones, who acted alongside Dixon in the TV series--asks if they've met before.)
 * Chris Langham, Arthur Dent from the 1979 Institute of Contemporary Arts stage production of Hitchhiker's, pops up as Prak in the finale of the Tertiary Phase.
 * Retcon: The entire Secondary Phase is just a dream Zaphod had, to cover for the Tertiary Phase opening with Ford and Arthur still being stranded on prehistoric Earth.
 * Sound Effect Bleep: Fit the Sixteenth from the Tertiary Phase. The book Life, The Universe And Everything on which the Phase was based featured an award for "The Most Gratuitous Use of the Word Fuck in a Serious Screenplay"; since it was scheduled to be broadcast at 6.30 pm the word was still uttered by the actor but completely masked by the sound of a starship engine.
 * Sound Effect Bleep: Fit the Sixteenth from the Tertiary Phase. The book Life, The Universe And Everything on which the Phase was based featured an award for "The Most Gratuitous Use of the Word Fuck in a Serious Screenplay"; since it was scheduled to be broadcast at 6.30 pm the word was still uttered by the actor but completely masked by the sound of a starship engine.