The Salmon of Doubt is the third and final Dirk Gently novel, written by Douglas Adams and published in 2002. The book was never finished, as Adams died unexpectedly in 2001. The unfinished work was published alongside several other pieces of Adams' writing in The Salmon of Doubt: Hitchhiking the Galaxy One Last Time.
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The prologue, set in a distant future long after the extinction of the human race, features a man called Dave hang-gliding over what used to be California, now renamed "Daveland".
A few weeks after the events of The Long Dark Tea-Time of the Soul, Dirk Gently is approached by a client to find the rear half of a cat - the front half being in perfect health and completely oblivious to the laws of biology or physics. He turns them down. Later he calls Kate Schechter and relates to her that he has been receiving fewer clients recently, and, in testing that the phone line was still working, he had started calling the number only to find that he himself was answering it on the other end.
Shortly after this, Dirk discovers that someone is paying him large amounts of money each month. He deduces that the variances in the amounts is due to the person is constantly travelling through different countries, and as a result they are paying the same amount in different currencies, with different exchange rates. Feeling he should do something to earn this money, he elects to follow a person at random.
Meanwhile, someone steals a car and a rhinoceros called Desmond unexpectedly winds up in downtown Los Angeles, where he subsequently panics and goes on the rampage.
A fax by Adams' secretary mentions hints at Adams' later intentions, featuring passage "through the nasal membranes of a rhinoceros" (possibly a reference to the character Desmond the rhinoceros) "to a distant future dominated by estate agents and heavily armed kangaroos."
- One scene in the novel depicts Dirk Gently using his "Holistic Navigation" technique and following a man whose description of Ford Prefect of Douglas Adams' The Hitchhiker's Guide to the Galaxy series.
- BBC Radio 4 had commissioned a third radio series based on the unfinished novel, to be adapted by writer Kim Fuller; however, these plans were scrapped in favour of the BBC TV adaptation.
- BBC plans Dirk Gently TV series : News 2009 : Chortle : The UK Comedy Guide