The original intention of The Salmon of Doubt was a Dirk Gently novel. 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 USA. 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."
The version in the published book is the strongest content from several unfinished drafts that were written.