Summary:
Arthur Dent has hitchhiked through the galaxy and is dropped off on a planet in a rainstorm. He realises that he appears to be in England on Earth, even though he saw it destroyed by the Vogons. While he has been gone for several years, it appears only a few months have pa**ed on Earth. He manages to hitch a ride with a man named Russell, who is driving home his sister Fenchurch (Fenny for short). Russell explains that she had become delusional after worldwide ma** hysteria over the "hallucinations with the big yellow spaceships" (Vogon ships). Arthur also learns that all the dolphins disappeared shortly after that event. Arthur becomes curious about Fenchurch, but they reach his home before he can ask more questions. Inside his still-standing home, Arthur finds a gift-wrapped bowl inscribed with the words "So Long and Thanks..." which he then uses for his Babel Fish. Arthur considers that Fenchurch is somehow connected to him and to the Earth's destruction, and finds that he still has the ability to fly whenever he lets his thoughts wander.
After putting his life in order, Arthur tries to find out more about Fenchurch. He catches her hitchhiking and he learns more about her. He obtains her phone number but loses it. He finds her home when he locates the cave he had lived in after crashing onto prehistoric Earth with the Golgafrinchans: her flat is built on the same spot. As they talk they find more circumstances connecting them. Fenchurch reveals that, moments before her hallucinations, she had an epiphany while sitting in a café about how to make everything right, but then blacked out. Ever since, she has not been able to recall it. After noticing that Fenchurch's feet do not touch the ground, Arthur teaches her how to fly and together they make love in the skies over London.
The two travel to California to see John Watson, an enigmatic scientist who purports to know the cause of the dolphins' disappearance and who eschewed his original name in favour of "Wonko the Sane" due to harbouring the belief that the entire world's population save himself has gone mad. Watson shows the couple a bowl with the words "So long and thanks for all the fish" inscribed on it that they all own and encourages them to listen to it. They learn from the bowl's audio message that the dolphins, aware of the Vogons, left Earth for an alternate dimension but not before replacing the destroyed Earth with a new version and transporting everything to it as a way of saving humans. Arthur explains to Fenchurch about hitchhiking across the galaxy, after which she insists that she wants to see it as well. They plan to hitchhike on the next pa**ing spaceship.
Concurrent to these events, Ford Prefect discovers that during an update of the "Hitchhiker's Guide", his previous entry for Earth, "Mostly harmless", has been replaced with the volumes of text he wrote during his research. Recognising 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 centre of London and causing a panic. In the chaos, Ford meets up with Arthur and Fenchurch and together they commandeer the robot's ship. Arthur takes Fenchurch to the planet where God's Final Message to His Creation is written, and they happen across Marvin, who, because of 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 Fenchurch's help, reads the Message ("We apologise for the inconvenience"), smiles, utters the final words "I think... I feel good about it," and dies happily.