Earth was a giant supercomputer designed to find the Ultimate Question of Life, the Universe and Everything. Designed by Deep Thought and built by the Magratheans, it was commonly mistaken for a planet, especially by the ape descendants who lived on it. It was situated far out in the uncharted backwaters of the unfashionable end of the Western Spiral Arm of the Galaxy.

Unfortunately, the Earth was destroyed by the Vogons five minutes before the program was to be completed. The Vogons were sent by the psychiatrist Gag Halfrunt, who thought his profession would cease if the Question were known. Later on, the Earth reappeared but all forms of the Earth were later demolished.

The only two humans to survive the Earth's destruction were Arthur Dent and Trillian.

Galactic Sector ZZ9 Plural Z Alpha is a galactic sector containing Earth, Barnard's Star, and probably other nearby systems. The "Plural" designation may indicate probability problems; after Fenchurch disappeared during a hyperspace jump, it was pointed out to Arthur Dent that the small print advised against hyperspace travel for those born in Plural sectors. This is apparently common knowledge throughout the rest of the galaxy.

The universe is an unexplainable and complicated place. It's mind-witheringly big and stupefyingly old. Add as a result of the cumulative effects of the more advanced forms of multidimensional travel, and it can be clearly seen to be having a rough time of it. So it can be forgiven for being just a little unstable. And an example of this instability that immediately springs to mind would be the cracks in the Fifth Dimension of Probability that are the Plural Zones. And that's just what Plural Zones are: instabilities in space-time.

The effects of the Plural Zones on the beings and objects that inhabit and originate from them, for the most part, remain largely unnoticed, barring those which sometimes occur after the following three events:

1. The sudden destruction of said objects by, say, an overzealous hyperspace planning council,
2. Hyperspace travel itself, or
3. Outside meddling from beings that know about these sort of things The results of the infliction of the first point can be likened to abducting a rubber duck from an infinite tube full of more rubber ducks. Take the one out of the picture, and another one will most likely just pop up from somewhere else in the Whole Sort of General Mish Mash.

“A computer of such infinite and subtle complexity that organic life itself shall form part of its operational matrix”—Deep Thought on the Earth.

When the supercomputer Deep Thought announced the Ultimate Answer as 42 after seven and a half million years of thought, it challenged Phouchg and Loonquawl to say what the Ultimate Question actually was. When they could not, Deep Thought designed a supercomputer that could give the Question, giving it its future name of the Earth. The Magratheans were then hired to build the Earth, using a method of sucking matter through white holes to construct the vast planet-like computer. A Magrathean designer named Slartibartfast had designed the fjords of Norway, for which he won an award.

The Golgafrinchan Ark Fleet Ship B crash-landed on Earth, leaving Arthur Dent and Ford Prefect stranded in time, two million years before Arthur's birth. Not long after the Golgafrinchans crashed in prehistoric Great Britain, the native australopithecines began dying off. Finding the climate too chilly, the Golgafrinchans up and moved to warmer climes in Africa where they began to multiply, replacing all the hominids native to the planet. This, of course. threw a spanner into the delicate ten-million-year experiment.

The psychiatrist Gag Halfrunt thought that his career would be endangered if the Question were known. He colluded with Zarniwoop van Harl and bribed Zaphod Beeblebrox to go along with the plan to destroy Earth. He got the Vogons to destroy the supercomputer with the apparent excuse of building a hyperspace bypass. Prostetnic Vogon Jeltz destroyed the Earth five minutes before the Question was due to be announced, thereby destroying ten million years worth of work.

Arthur discovered that the entry for "Earth" in the Hitch Hiker's Guide to the Galaxy, which formerly had been edited down to "Mostly Harmless", had been replaced with Ford's original full version. They headed towards Earth independently, Arthur arriving first. After landing in a field in Somerset, Arthur tried to hike a lift to Cottington, to see if his house still existed. Along the way, he mets Rob McKenna, a man who complained about the rain but actually was a rain god, before realising he has hitched a lift the wrong way. He got out, and hitched a lift with Russell, whose sister Fenchurch was out cold on a back seat of the car. Arthur was instantly smitten, and asked about her. Russell claimed that she was mad, and had been ever since the "hallucinations" a year ago—actually the Vogon Constructor Fleet. Fenchurch and Arthur got together and began a relationship, as Fenchurch had found Arthur's copy of the Guide. Finding that her "madness" was actually due to a brief flash of insight at the moment of the Vogon appearance/disappearance, the susbstance of which hd disappeared from her consciousness. The only physical effect was that her feet no longer touched the ground, leaving her—as Arthur put it—"an inch taller." She and Arthur travelled to California to meet John Watson (a.k.a. Wonko the Sane), an oceanologist who could help them discover what had happened to the dolphins).

Meanwhile, Ford was stuck in a bar with a large bill, which he avoided paying by promising to write an entry for the bar in the Guide. On the streets, he was asked by a hooker whether he is "rich", and said that he might be, being owed several years back pay for writing two words. He showed the two words—"Mostly Harmless"—to the hooker, and was shocked to see the guide updating this to his full entry. He then decided to go to Earth himself, where he reunited with Arthur and met Fenchurch, the two of whom had just returned to England from California.

Leaving the replacement Earth, Arthur and Fenchurch travelled to see God's Final Message To His Creation, written in fire in letters thirty feet high on the far side of the Quentulus Quazgar Mountains in the land of Sevorbeupstry on the planet of Preliumtarn, which orbits the star Zarss, which is located in the Grey Binding Fiefdoms of Saxaquine. After that, they had plans to tour the Galaxy, but due to her origin in a Plural Zone, she vanished during a routine starship hyperspace jump.