One of the themes of the book is the relationship between astronomy and astrology.
Arthur Dent has continued exploring the universe. He believes that he lives a charmed life, unable to die, due to his encounter with Agrajag; he knows that he is destined to visit Stavromula Beta and kill Agrajag there before he dies, but nobody has ever heard of Stavromula Beta. As a result of a shuttle crash, he finds himself on planet Lamuella, where people worship a man named Almighty Bob and becomes a sandwich maker. After a little while, Trillian comes to see him. Trillian presents Arthur's daughter, Random, to him, for him to look after while Trillian continues her career as a galactic journalist.
After a month or two, Random runs away, taking with her a parcel mailed by Ford Prefect in care of Arthur. With the help of the Guide Mark II, Random steals a very nice spaceship from Ford, who has come to Lamuella for his parcel. Random heads to Earth. Ford and Arthur find their way to a bar and grill where they buy EP's (Elvis Presley's) spaceship and head to Earth.
Trillian has been attempting to cover a war occurring elsewhere in the galaxy, but a time anomaly results in a battleship for one of the warring factions (the Grebulons) appearing in Earth space where it cannot contribute to the war. Trillian visits the ship and is disappointed there is no news story. The Grebulon commander has been watching television broadcasts captured from Earth and asks Trillian to read his horoscope, which she attempts to do, allowing for Earth as an astrological body. The horoscope is bad, and Trillian realises that the Grebulon may be preparing to destroy Earth to improve his horoscope. Trillian travels to Earth to warn Random and her friends.
Ford and Arthur get to Earth in time to discover Trillian and Tricia McMillan fighting over a psychotic Random in a club called "Stavro's." Trillian desperately attempts to convince Random to leave Earth before it is destroyed, but Arthur claims Earth cannot be destroyed while he is on it because he must kill Agrajag on Stavromula Beta before he dies. However, Random then shoots a man emerging from the gents' toilet. The man wrongly blames Arthur for his death, and Arthur learns that the club is owned by a man called "Stavro Mueller Beta" and thus the man was the final incarnation of Agrajag: Arthur's time loop is closed and he can die. As the realization sinks in, Earth is destroyed by the Grebulon commander.
Ford, Arthur, Trillian, Tricia, Random, and every other human are killed, and forever dead on Earth. Ford and Arthur, at the end of the book, are happy and relieved that "it" is over; Ford dies laughing wildly, and Arthur dies with relief. It is unknown how Tricia, Trillian, and Random feel when they die.
The book finally reveals that the Guide Mark II is a device created by the Vogons to arrange the destruction of Earth to occur in all possible probability spaces, thus preventing Earth from being restored by the dolphins. Prostentic Vogon Jeltz finally crosses the task off his to-do list. The Grebulon Commander's fortunes do not improve by destroying Earth and he mildly regrets that there is nothing on television anymore.
Douglas Adams frequently expressed his disdain for this ending in retrospect, claiming that it was too depressing and came about as the result of him having "a bad year;" "People have said, quite rightly, that Mostly Harmless is a very bleak book. And it was a bleak book. I would love to finish Hitchhiker on a slightly more upbeat note, so five seems to be a wrong kind of number; six is a better kind of number." He had planned to write a sixth book to undo this 'mistake', but never got around to it before his death. Zaphod Beeblebrox is mentioned but does not appear in Mostly Harmless, and therefore presumably is not on Earth, and thus survives; the official sequel And Another Thing..., written by Eoin Colfer, undoes the ending regretted by Adams by having Zaphod rescue the group in the nick of time.
With grateful thanks
to Sue Freestone and Michael Bywater
for their support, help and constructive abuse
Anything that happens, happens.
Anything that, in happening,
causes something else to happen,
causes something else to happen.
Anything that, in happening, causes itself
to happen again, happens again.
It doesn't necessarily do it
in chronological order, though.
- Gail Andrews
- Eric Bartlett
- Colin the Security Robot
- Vann Harl
- Old Thrashbarg
- Guide Mark II
- The Leader
- Zarquon (mentioned)
- Woody Allen (mentioned)
- Mo Minetti (mentioned)
- Gary Andress (mentioned)
- Zaphod Beeblebrox (mentioned)
- Stavro (mentioned)
- Karl (mentioned)
- Elvis Presley (mentioned)
- St Antwelm (mentioned)
- Stagyar-zil-Doggo (mentioned)
- Fenchurch (mentioned)
- Emily Saunders (mentioned)
- Roopy Ga Stip (mentioned)
- Prince Gid of the Soofling Dynasty (mentioned)
- Princess Hooli of Raui Alpha (mentioned)
- Great Green Arkleseizure (mentioned)
- Zarquon (mentioned)
- Kirp (mentioned)
- Grarp (mentioned)
- The Baker (mentioned)
- Strinder the Tool Maker (mentioned)
- Drimple (mentioned)
- Almighty Bob (mentioned)
- The Hitchhiker's Guide to the Galaxy
- The Hitchhiker's Guide to the Galaxy Mark II
- No.3 Gauge Prising Tool
- Dine-O-Charge Credit Card
- Infinite Improbability Drive (mentioned)
- Breathe-o-Smart (mentioned)
- SlumpJet (mentioned)
- Saquo-Pilia Hensha
- Pintleton Alpha
- The Domain of the King Bar & Grill
- Stavro Mueller Beta
- Arkintoofle Minor (mentioned)
- University of Maximegalon (mentioned)
- Santraginus V (mentioned)
- Frastra (mentioned)
- Damasco (mentioned)
- Pluto (mentioned)
- Neptune (mentioned)
- Maximegalon Institute of Slowly and Painfully Working Out the Surprisingly Obvious (mentioned)
- Viv (mentioned)
- OhWell (mentioned)
- Gagrakacka Mind Zones (mentioned)
- Poffla Vigus (mentioned)
- Bjanjy Territories (mentioned)
- Stavromula Beta (mentioned)
- Betelgeuse V (mentioned)
- Fanalla (mentioned)
Races and species
- Nowhattian Boghog
- Pikka birds
- Hingefreel (mentioned)
- Frastran (mentioned)
- Riktanarqal (mentioned)
- Danqued (mentioned)
- Perfectly Normal Beasts
- It Was You and Your Planets
- Practical Parenting in a Fractally Demented Universe (mentioned)
The profoundly bleak ending of Mostly Harmless attracted anger from critics and fans, and Douglas Adams later admitted that he did not like the ending and blamed it on himself being extremely depressed at the time. Adams strongly implied that he might like to revisit the series and write a final book with a more upbeat ending, and commented that the annihilation of Earth and all of the major characters in every possible timeline would simply ensure that "he would not have to work out how they would meet up again at the start of the next book."
Since the audio adaptation was made after Douglas Adams' death, and thus it was known that there was no possibility of an official sequel, an altered upbeat ending was added to the audio version of Mostly Harmless. Earth is still destroyed by the Grebulon commander, but it is then revealed that the Babel Fish is a close relative of the Dolphin and shares its ability to travel in possibility space. All of the major characters are carrying Babel Fish is their ears, who rescue them by transporting them to the Restaurant At The End Of The Universe; here they are reunited with an earlier instance of Marvin (who is working parking cars) and Fenchurch (who, although she was not involved in the plot of Mostly Harmless, did still have a Babel Fish). Arthur discovers that beyond the car park of the Restaurant lies "thousands of shimmering blue lagoons" - the Dolphins' final destination - and flies off together with Fenchurch.