The Salmon of Doubt: Hitchhiking the Galaxy One Last Time is a posthumous collection of previously published and unpublished material by Douglas Adams.

The plot, set a few weeks after the events in The Long Dark Tea-Time of the Soul, involves Dirk Gently refusing to help find the missing half of a cat, receiving large amounts of money from an unknown client, and then flying to the United States.

Dirk pays a visit to Kate Schechter (who had first appeared in The Long Dark Tea-Time of the Soul) and tells her that prior to the potential client, he had been so bored that he had started a habit of dialling his own phone number and discovered he'd answered his own calls.

A faxed summary reprinted before the text mentions travelling "through the nasal membranes of a rhinoceros, to a distant future dominated by estate agents and heavily armed kangaroos."

Adams said that while he originally planned on writing a third Dirk Gently book, the ideas which he had for it would have fitted better into another Hitchhiker's book: "A lot of the stuff which was originally in The Salmon of Doubt really wasn't working", and he planned on "salvaging some of the ideas that I couldn't make work in a Dirk Gently framework and putting them in a Hitchhiker framework... and for old time's sake I may call it The Salmon of Doubt.

The book ends with the order of events for the memorial service that was held for Adams in September 2001, at St Martin-in-the-Fields church in London.