Summary:
Arthur Dent plans to sightsee across the Galaxy with his girlfriend Fenchurch, but she disappears during a hyperspace jump, a result from being from an unstable sector of the Galaxy. Depressed, Arthur continues to travel the galaxy using his biological donations to DNA banks to fund his travels, and knowing that he cannot die until he visits Stavromula Beta, as told to him by the insane Agrajag whom had been repeated k**ed in various ways by Arthur before being reincarnated. During one trip, he ends up stranded on the homely planet Lamuella, and decides to stay to become a sandwich maker for the local population.
Meanwhile, Ford Prefect has returned to the offices of the Hitchhikers' Guide, and is annoyed to find out the original publishing company, Megadodo Publications, has been taken over by InfiniDim Enterprises, which are run by the Vogons. Fearing for his life, he escapes the building, along the way stealing the yet-published, seemingly sentient Hitchhiker's Guide Mk. II. He goes into hiding after sending the Guide to himself, care of Arthur, for safekeeping.
On Lamuella, Arthur is surprised by the appearance of Trillian with a teenage daughter, Random Dent. Trillian explains that she wanted a child, and could use the only human DNA she could find, thus claiming that Arthur is Random's father. She leaves Random with Arthur to allow her to better pursue her career as an intergalactic reporter. Random is frustrated with Arthur and life on Lamuella; when Ford's package to Arthur arrives, she takes it and discovers the Guide. The Guide helps her to escape the planet on Ford's ship after Ford arrives on the planet looking for Arthur. Discovering Random, the Guide, and Ford's ship missing, the two manage to find a way to leave Lamuella and head for Earth, where they suspect Random is also heading to find Trillian. Ford expresses concern on the Guide's manipulation of events.
Reporter Tricia McMillan is an alternate version of Trillian living on Earth who never took Zaphod Beeblebrox's offer to travel in space. She is approached by an extraterrestrial species, the Grebulons, who have created a base of operations on the planet Rupert , a recently discovered tenth planet in the Solar System. However, due to damage to their ship in arriving, they have lost most of their computer core and their memories, with the only salvagable instructions being to observe something interesting with Earth. They ask Tricia's help to adapt astrology charts for Rupert in exchange for allowing her to interview them. Tricia performs the interview but finds that the resulting footage looks like the whole thing was fake. She is called away from editing the footage to report on a spaceship landing in the middle of London.
Tricia finds Random leaving the ship and yelling at her, believing her to be her mother. Arthur, Ford, and Trillian soon arrive and help Tricia to calm Random down and take her to a nearby bar to talk to her away from the chaos of the spacecraft. Trillian tries to warn the group that the Grebulons, having become bored of their mission, are about to destroy the Earth, but Random disrupts their discussion, brandishing a laser gun she took from her ship. Arthur, still believing he cannot die, tries to calm Random's nerves, but a distraction causes her to flinch and fire at Arthur. He ducks in time, but the shot hit a man behind him, sending the bar into a panic. Arthur tends to the man and learns the man is another reincarnation of Agrajag, and discovers the bar is named "Stavro Mueller - Beta". Arthur begins to laugh as a larger panic brews up outside.
The Grebulons, having determined that by removing Earth from the astrological charts will improve their horoscopes, destroy the Earth and k**ing everyone upon it. Meanwhile, it is revealed that the Guide Mk. II was created by the Vogons with the ability to see into every possibility as to manipulate events so that every version of the Earth from all realities was destroyed. With its mission complete, the Guide collapses into nothingness.