The Salmon of Doubt is an unfinished manuscript by Douglas Adams, left in that state after the author sadly hitched a lift to the Great Beyond. Its exact nature is, and forever will remain, unclear: it started life as a third Dirk Gently book, but the author later implied that it wasn't working out that way, and that he might change it into a sixth Hitchhiker's Guide (partly to end the series on a happier note after the bitter fifth book). However, it contains strange passages that don't fit neatly into either series, such as an incongruous, uncharacteristic account of a Los Angeles carjacking and an account of a man named Dave, hang-gliding over a post-apocalyptic woodland.
Several chapters from the manuscript (primarily those featuring Dirk Gently) have been compiled and published in The Salmon of Doubt: Hitchhiking the Galaxy One Last Time. This book is largely composed of essays, anecdotes, and other non-fiction writing from Douglas Adams, and merely takes its name from - and includes sizable fragments of — the manuscript at the end of the book. The short stories "Young Zaphod Plays It Safe" (a short prequel to The Hitchhiker's Guide to the Galaxy focusing on the character of Zaphod Beeblebrox) and "The Private Life of Genghis Khan" are included, as well.
As yet, no one has attempted to finish the book; whether it ever receives the Edwin Drood treatment remains to be seen.
The story, such as it is, is cryptic and surreal even by Adamsian standards. It begins with Dirk Gently, the con artist / private detective last seen in The Long Dark Tea-Time of the Soul, being asked to investigate the disappearance of the back half of a woman's cat (the cat, strangely, is just fine, and seems not to have noticed that he's only mostly there). Shortly thereafter, Dirk begins to receive a series of enormous payments from an unseen client, although he's done nothing to earn it. Convinced that he has to do something to justify this, he follows a ginger-haired actor (fan speculation identifies him as Ford Prefect) onto a flight bound for Los Angeles. Dirk then meets a rhinoceros named Desmond on the back of a truck. The story is largely fragments, and consists of various chapters from various drafts that Adams wrote at different times and because of this, there's no specific point where the text breaks off mid-thought, the story just sort of stops at the end of a chapter.
- After the End: Dave is implied to live in the far future, in a world where nature has reclaimed most of the planet.
- All Hail the Great God Mickey!: Dave has a toy or ornament of Kermit the Frog on his shrine to Saint Clive.
- Ambiguously Human: Dave and his society, due to it mentioning that humanity died out long ago. There's a throwaway line in modern day about scientists teaching kangaroos to talk, implying that they might rise up to replace us.
- Animal Is the New Man: As said above, it's implied that kangaroos might replace us.
- Arc Words: "Gusty Winds may exist" (possibly referring to Dirk's client's half-missing cat).
- Audio Adaptation: The BBC commissioned one but scrapped it to make the Dirk Gently TV series.
- Batman Gambit: It's strongly suggested that Dirk's mystery client knows exactly how Dirk will react to the payments, right down to which person he will randomly choose to follow. It's also possible that this is justified by Time Travel.
- Call-Back: To the chapter on the Northern White Rhino in "Last Chance to See" where the author described how the rhino's nasal passages are larger than its brain.
- Cool Gate: Douglas Adams's plot summary mentions Dirk Gently travelling "through the nasal membranes of a rhinoceros, to a distant future dominated by estate agents and heavily armed kangaroos", while in the chapter about DaveLand, a mysterious "Way of the Nostril" is being mentioned as the basis of DaveLand's existence. The implication seems to be that Dave possibly discovered a way to travel 1.2 million years into the future that somehow involves passing through a rhinoceros's nose.
- Egopolis: Dave lives in DaveLand, complete with a river Dave. He had renamed the Pacific Ocean as the Dave Ocean before deciding to call it the Karen Ocean after Karen Carpenter.
- Flat-Earth Atheist: One of Dirk's clients used a medium to contact her dead husband but all he would say was that he didn't believe in any of that spiritualist nonsense.
- Follow That Car: Dirk says "Follow that cab" to a taxi driver when chasing his quarry, starting a long rant from the cabbie that nobody's ever said that to him before but that taxis on TV seem to have no other function.
- Humanity's Wake: We suddenly died out 1.2 million years before Dave's time.
- Invisible Anatomy: Dirk is hired to find the back half of a cat. The cat's front end can still walk and stand as if it has four legs.
- Missing Steps Plan: One of the reasons Dirk refuses to look for the back half of a cat is that he has no idea how to reattach it if he does find it.
- Rhino Rampage: Desmond, in chapter nine.
- Sequel Goes Foreign: Dirk goes to America.
- Soundtrack Dissonance: After a woman has her car stolen, the narrator says he wishes the radio of a passing convertible was playing "How does it feeeeel" but it was actually playing "Sunday Girl" by Blondie which wasn't even remotely appropriate.
- Uplifted Animal: The cabbie complains about scientists teaching kangaroos to talk.