So Long, and Thanks for All the Fish (1984, ISBN 0-345-39183-7) is the fourth book of the Hitchhiker's Guide to the Galaxy series written by Douglas Adams. Its title is the message left by the dolphins when they departed Planet Earth just before it was demolished to make way for a hyperspatial express route, as described in The Hitchhiker's Guide to the Galaxy. The phrase has since been adopted by some science fiction fans as a humorous way to say "goodbye" and a song of the same name was featured in the 2005 film adaptation of The Hitchhiker's Guide to the Galaxy.
After putting his life back in order, Arthur goes about trying to find more information about Fenny. He manages to catch her hitchhiking, and talks with her more in a pub, learning more about her unusual circumstances, but before he can get a number or address from her, she has to leave. He happens upon her home when he attempts to locate the cave he had previously lived in after crashing onto prehistoric Earth with the Galgafrinchians, Fenny's flat built on the same spot. As the two talk and find a connection between each other, including her having a similar bowl to Arthur's, Fenchurch reveals that moments before the hallucinations with the yellow ships, she had an epiphany about how to make everything right while sitting in a cafe, but then blacked out; even since then, she cannot recall that epiphany. After seeing that Fenny's feet do not touch the ground, Arthur teaches her how to fly, and together they make love over the London skies.
Together, they conventionally travel to California to see John Watson, aka "Wonko the Sane", a scientist who purports to know the cause of the dolphins' disappearance. He points out that he, too, has a similar bowl as Arthur and Fenny, and encourages them to listen to the bowl. Upon doing this, they learn from an audio message in the bowl that the dolphins, aware of the destruction of Earth by the Vogons, left the Earth to an alternate dimension but not before replacing the destroyed Earth with a new version, transporting everyone and everything to it as a way of saving the human species. After realizing the truth, Arthur explains what he has experienced in the last several years in hitchhiking across the galaxy to Fenny, who insists that she wants to see it all as well. They make plans to hitchhike out on the next passing spaceship.
Concurrent to these events, Ford Prefect discovers that during an update of the "Hitchhiker's Guide", that his previous entry for Earth, "Mostly harmless", has been replaced with the volumes of text he wrote during his research. Recognizing that something is strange, Ford begins to hitchhike across the galaxy to reach Earth, eventually using the ship of a giant robot to land in the center of London and causing a panic. In the chaos, Ford meets up with Arthur and Fenny, and together commandeer the robot's ship. Arther takes Fenny to the planet where God's Final Message to His Creation is written, and happen across Marvin, who, due to previous events, is now approximately 37 times older than the known age of the universe, and is barely able to continue. Marvin, with Arthur and Fenny's help, reads the Message, "WE APOLOGIZE FOR THE INCONVENIENCE", smiles, and dies happy.
The novel has a very different tone to the previous books in the series. Partly this is because it is a romance, and partly because the book jumps in time more than usual. Douglas Adams even humorously puts a side-story in it. Perhaps most notably, there is very little space-travel in the entire book, with Arthur only leaving the new Earth in the final chapters. Adams' editor Sonny Mehta moved in with the author to ensure that the book met its (extended) deadline. As a result, Adams later stated that he was not entirely happy with the book, which includes several jarring authorial intrusions, which fellow author Neil Gaiman described as "patronising and unfair".
The book also reflects a significant shift in Adams' view of computers. In the previous books, computers had been portrayed quite negatively, reflecting Adams' then views on the subject. However, between the writing of Life, The Universe and Everything and So Long and Thanks for all the Fish, his attitude toward technology changed considerably. Having been taken along to a computer fair, he became enamoured with the first model of the Macintosh, the start of a long love-affair with the brand (he claimed to have bought two of the first three Macintosh in the UK - the other being bought by his friend Stephen Fry). In So Long And Thanks For All The Fish, Arthur Dent purchases a computer for the purpose of star mapping; Adams makes only one disparaging comment about this decision.
Betsy Shorb reviewing for the School Library Journal said that "the humor is still off-the-wall but more gentle than the other books. The plot is more straight forward and slightly less bizarre than its predecessors".