''Mostly Harmless'' (1992) is the fifth installment in the [[TrilogyCreep increasingly inaccurately-named]] ''Literature/TheHitchhikersGuideToTheGalaxy'' trilogy, and the last written by franchise creator Creator/DouglasAdams himself. Set several years after the fourth one, ''Literature/SoLongAndThanksForAllTheFish'', its plot divides into three distinct branches. The first concerns the series protagonist, hapless Englishman Arthur Dent, as he tries to find a new, Earth-like home to settle down on after the unexpected erasure of his soul-mate, Fenchurch; he eventually builds a sort of life for himself as resident sandwich-maker for the primitive villagers on the planet Lamuella (but not for long).

The second concerns Ford Prefect and his ill-conceived attempts to save the ''Guide'' Corporation from [[CorruptCorporateExecutive corporate sleaze]] Vann Harl and Infini-Dim Enterprises, a bureaucratic nightmare run by the Vogons that wants to pervert the spirit of ''The Hitchhiker's Guide to the Galaxy'' for profit. The third concerns Trillian, who reinvents herself as an interplanetary anchorwoman and bears Arthur's child, Random Dent, who is absolutely nothing like her mild-mannered father.

This book is by far [[DarkerAndEdgier the darkest of the series]], which had hitherto been very upbeat, due to a bad case of CreatorBreakdown.

Preceded by ''Literature/SoLongAndThanksForAllTheFish''. Succeeded by ''Literature/AndAnotherThing''. Has next to nothing to do with the Connecticut sketch comedy ''Series/MostlyHarmless'', but is definitely related to the ''Roleplay/MostlyHarmless'' Roleplay [[Website/LiveJournal LJ]] group.

!!''Mostly Harmless'' provides examples of the following tropes:

* AlternateUniverse
* AntiAdvice: Arthur Dent asks a soothsayer how he should live his life, and her counsel boils down to "Here's my autobiography. Read it, and do the opposite of what I did."
* {{Astrologer}}: Gail Andrews
* AxCrazy: Random Dent
* BrattyTeenageDaughter: Random. Justified as her mother isn't really interested in raising her, and she's basically been dumped with her unsupecting father on boring backwater planet Lamuella.
* BuffySpeak: The Guide calls the sum total of all possible universes/timelines the Whole Sort of General Mish Mash, or WSOGMM for short.
* BusCrash: Fenchurch
* CerebusSyndrome
* TheChessmaster: The Guide Mark II.
* CorruptCorporateExecutive: Vann Harl
* CrapsackWorld: [=NowWhat=], an alternate Earth which Arthur visits on his travels. It's so bad that the ghastly smile on the president's picture is due to the fact he'd just shot himself, and every inhabitant has only one ambition: leave.
* DarkerAndEdgier: Than the previous books in the "trilogy".
* DownerEnding: [[spoiler:All AlternateUniverse Earths are destroyed, and [[RocksFallEveryoneDies 80% of the main cast dies]].]] See CreatorBreakdown. [[spoiler: ...or so it seems, until the sixth book reveals that, at Random's request, they're placed into a LotusEaterMachine by the Guide Mk. II and given one last shot at survival.]]
* DroppedABridgeOnHim: [[spoiler:Poor Fenchurch. At least the radio adaptation has a DeusExMachina at the end that spares her.]]
* RocksFallEveryoneDies: [[spoiler:Or more like "Earth Gets Obliterated, Everyone Dies".]]]]
* EldritchAbomination: The Guide Mk. II is a pandimensional ''thing'' of limitless power (it can make anything happen simply by arranging events in the past to bring about a given future event) that has the appearance of a black bird. [[AFormYouAreComfortableWith Sometimes]].
* ElvisHasLeftThePlanet
* FauxAffablyEvil: Vann Harl [[spoiler:(actually Zarniwoop, as described in the second book)]], [[CorruptCorporateExecutive corporate bastard extraordinaire.]] By this point in the series, the ''Guide'' had come to represent freedom from the norm, and thus, both his transformation of the ''Guide'' HQ from a dadaesque palace of fun to a drab corporate office complex and his attempt to misuse [[TimeTravel time-traveling technology]] to profit infinitely from a single sale are seen as gruesome sins in this series' world. However, he is ever so utterly suave and in-control... though we know he is loathsome, it's hard not to like him. Just a little.
%% For the last freaking time, Fanon Discontinuity is a subjective trope and goes in the YMMV namespace. This is a ban warning for the next troper who adds it back.
* GambitRoulette: "Temporal reverse-engineering", the special feature of the Guide Mark II; give it a goal, and it will turn out to have already been acting to bring it about while also working on somebody else's future agenda ''and'' ensuring you give it the goal it wants.
* GenreShift: This book isn't funny. Mostly it's just [[MindScrew creepy and strange]].
* ItRunsOnNonsensoleum: The series' phlebotinum runs entirely on RuleOfFunny. In this book, there's an anecdote about a race whose interstellar craft run on bad news, the only thing in the universe that travels faster than light. (Their ships don't work very well, and somehow nobody's ever pleased to see them.)
* LittleMissBadass: Random (at the very least, she ''tries'' to be).
* MarriedToTheJob: Trillian is much more interested in her career than raising her (clearly emotionally damaged) daughter. Granted some of it isn't her fault, temporal induced aging and all, but it comes across as enormously selfish that she doesn't even ''consider'' a job that would let her stay in Random's life.
* MegaCorp: The ''Hitchhiker's Guide'' Corporation and Infini-Dim Enterprises.
* MindScrew:
** Ford's voyage into the four-dimensional ''Guide'' online network.
** The scene in which Arthur visited the man in the village filled with poles. Said man can walk ''to the horizon and back'' in a single step, and can step off one of the poles (each one is 40 feet tall, mind you) and live.
* MundaneMadeAwesome: Arthur's epic sandwich-making skills.
* TheOperatorsMustBeCrazy:
-->''...the third event, which was a rampaging mob of long-distance telephone operators who had got so twisted with having to say, all day and every day, "Thank you for using BS&S" to every single idiot who picked up a phone that they had finally taken to the streets with trash cans, megaphones and rifles.\\
In the ensuing days of carnage every single window in the city, rocket-proof or not, was smashed, usually to accompanying cries of "Get off the line, asshole! I don't care what number you want, what extension you're calling from. Go and stick a firework up your bottom! Yeeehaah! Hoo Hoo Hoo! Velooooom! Squawk" and a variety of other animal noises that they didn't get a chance to practice in the normal line of their work.\\
As a result of this, all telephone operators were granted a constitutional right to say "Use BS&S and die!" at least once an hour when answering the phone and all office buildings were required to have windows that opened, even if only a little bit.''
* PlotArmor: Arthur Dent has it, and it's explicitly justified. Remember Agrajag from ''Literature/LifeTheUniverseAndEverything''? The guy who kept getting reincarnated, and whom Arthur keeps accidentally killing? There's an incarnation of him that Arthur ''hadn't'' met at that point, and the narration explicitly states that until Arthur accidentally kills ''that'' incarnation of Agrajag, he literally cannot die. [[spoiler:Which he does, at the end of this book, with his own death (and everyone else's) following by mere seconds.]]
* ThePollyanna: Colin, a flying security robot that Ford meets (and names) when he revisits the ''Guide'' HQ. Ford reprogrammed him so he would always feel happy, no matter what happens, effectively turning him into the anti-Marvin.
* SatiatingSandwich: Arthur Dent, having undergone an existential crisis over his only skill being sandwich making, ends up the chief sandwich maker for the tribe on Lamuella, with several whole pages about this exalted position, and his divine sandwich-making skills earn him more respect than the village chief.
* [[SirNotAppearingInThisTrailer Sir Not Appearing in This Book]]: Zaphod Beeblebrox and Marvin. Well, at least Marvin has a good excuse.
* SleeperStarship: The Grebulons were on one until they crashed on the planet Rupert and lost their memories.
* [[spoiler:SparedByTheAdaptation: All of the main characters (including Marvin) in the radio play.]]
* StrandedWithEdison: Discussed but averted; when Arthur Dent is stranded on a planet with an Iron Age culture, he intitally thinks he can bring them civilisation, before realising that he doesn't actually know how to make anything. Except sandwiches...
* TookALevelInBadass: After spending the last four books only interested in his hedonistic lifestyle, Ford, with said hedonistic lifestyle threatened, manages to escape several security robots with [[NoodleImplements a towel and a shoe.]]
* TookALevelInJerkass: Trillian (see MarriedToTheJob, above).
* UnfortunateNames: If you heard someone refer to Random Dent, you would think they were talking about some '''random dent''' on the wall. Her full name is Random ''Frequent Flyer'' Dent, making it ''even more'' pathetic.
* UnwittingPawn: Almost every major character.
* VomitIndiscretionShot: Arthur makes the mistake of taking a deep breath before approaching the old woman in the cave on Hawalius (whose cave is surrounded by goat corpses and flies). The results are not pretty, and described in nauseating detail.
* YouCannotGraspTheTrueForm: Arthur is thoroughly disturbed by the man on the pole not making any sort of spatial sense.