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.
The original two seasons contain examples of:
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).
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.
Quandary Phase and Quintessential Phase, however, do not complete the pattern formed by the first three. The names were chosen because they are "less daunting, more memorable and are a bit easier to spell" than the correct Quaternary and Quinary.
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:
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.
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. It's Arthur.
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.)
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."
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.
Vengeful Vending Machine: Arthur Dent, sick of getting bad tea from the Nutri-Matic machine, gives it a lengthy lecture on the nature and history of good tea. The machine hijacks the starship's entire computing power to work on the problem, leaving the ship defenceless against a missile attack. Arthur gets his tea in the end, though.
Weird Trade Union: The Amalgamated Union of Philosophers, Sages, Luminaries and Other Professional Thinking Persons, which opposed using the computer Deep Thought to find the Answer to the Ultimate Question of Life, the Universe and Everything because, under law, the quest for Ultimate Truth was under their prerogative, and not the computer's. They even threatened to go out on strike, though they dodged the question of who, exactly, that would inconvenience.
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.
Comic Book Time: The original two series, broadcast from 1978 - 1980, were intended as a contemporary piece. While the narrative quickly left Earth and there is not much to date it, it can still be a bit jarring when the later three series have scenes on or in reference to Earth that make more modern cultural references, or include as common technology things that would not yet have been common or even have existed. Of course, when dealing with possibilities such as different versions of Earth existing across multiple planes of reality, one supposes that such things may be relative.
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."
Everybody Lives: All the core cast manages to survive at the very end of the series, including Trillian and her alternate self (who merge into a single being), Marvin (who was still under warranty when he expired), and Fenchurch (who had been waiting at Milliway's since she seemingly ceased to exist). And the Vogons fail to eradicate every Earth.
Oddly Named Sequel: The original two series and the later adaptation of "Life, the Universe and Everything" were released as the Primary, Secondary and Tertiary Phases. The adaptations of the last two books became the Quandary and Quintessential Phases.
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 All 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.