Listen, it's a tough universe. There's all sorts of people and things trying to do you, kill you, rip you off, everything. If you're going to survive out there, you've really got to know where your towel is.
Ford Prefect is the deuteragonist of the late Douglas Adams' Hitchhiker's Guide to the Galaxy novel series, as well as its radio, television, and film adaptations. He is an extraterrestrial researcher for the titular encyclopedia who, along with his human best friend Arthur Dent, have a series of adventures through multiple galaxies and dimensions.
In the TV and radio adaptations of the series, he is portrayed and voiced by British actor David Dixon. In the film adaptation of the first novel, he is portrayed by American rapper/actor Yasiin Bey, aka Mos Def.
Ford is introduced as a researcher for the titular encyclopedia from a small planet in the vicinity of Betelgeuse; to blend in with the human race, he took the name Ford Prefect - unaware that it was in fact the brand name of a car - and posed as an out-of-work actor from Guildford. He meets Arthur moments after transporting to Earth, when Arthur saves him from being run over; Ford mistakenly assumes that cars are the dominant form of life on Earth, and walks into one's path in an attempt to introduce himself. The two afterwards become close friends.
When Ford learns that the Vogons are going to destroy Earth, he shows up at Arthur's house - which is coincidentally about to be bulldozed - and persuades him to accompany him to a neighborhood pub. There, he reveals his true identity and the Earth's fate to a bewildered Arthur, and manages to stow away with him on a Vogon ship seconds before Earth is destroyed.
After Ford and Arthur are nearly executed by the Vogons, they stumble into the Heart of Gold, a spaceship piloted by Ford's "semi-cousin" Zaphod Beeblebrox and Tricia McMillan, aka Trillian, a woman whom Arthur had utterly failed to pick up at a party months earlier. Ford accompanies them to Magrathea in search of the answer to the question of life, the universe, and everything, and eventually learns that the earth was in fact a computer designed by the Supercomputer Deep Thought to discover the question behind the answer - "42" - only to be destroyed by the Vogons just as it was about to complete its task.
The Magrathean descendants of Deep Thought's engineers, who have evolved into the forms of laboratory mice, take Ford and the others hostage so they can kill Arthur and harvest his brain, believing it to contain the question. They are all saved at the last moment by Slartibartfast, the architect who designed Earth, who transplants them onto the new, "replacement model". At the end of the novel, Ford suggests that they have lunch at the Restaurant at the End of the Universe.
The Restaurant at the End of the Universe
At the titular restaurant, Ford joins the others in witnessing the universe's demise, which plays on a continuous loop every night. He and Arthur board Ark Fleet Ship B to once again flee galactic destruction, only to find that the ship's captain has no idea what's he's doing and the ship's main passengers, the entire population of the planet Golgafrincha, are intentionally trying to destroy the galaxy to rid themselves of its less useful inhabitants. The ship crash-lands on a planet inhabited by primitive humanoid creatures, which they eventually discover to be prehistoric Earth, circa 2,000,000 B.C., after finding a glacier "signed" by Slartibartfast; they then surmise that the strange creatures are in fact Neanderthals.
Ford and Arthur discover that the Golgafrinchians plan to eradicate the Neanderthals, who are too preoccupied with their recent discovery of mindless bureaucracy to discover fire, the wheel, or basic self-defense. They try to warn the Neanderthals, who ignore them in favor of playing a game of Scrabble. When one of the Neanderthals spells out "Forty-two", Ford realizes that they are part of Deep Thought's matrix, and tries to get Arthur to search his subconscious for the question, which he is convinced Slartibartfast implanted in his brain. After Arthur finds the question - "What do you get when you multiply six by nine?" - Ford realizes that Slartibartfast got the question wrong, as six multiplied by nine is actually 54. Ford then says that this explains why the universe is "a giant cock-up", and he and Arthur resign themselves to making the best of life on prehistoric Earth.
Life, the Universe, and Everything
A few years later, Ford takes Arthur through a space-time eddy in the form of a giant sofa, and they go forward in time to a few days before the Earth is destroyed. As they arrive, a horde of robots materialize at Lord's Cricket Grounds and steal the Ashes. Slartibartfast arrives shortly afterwards, asking Ford and Arthur for their help. in stopping the robots from stealing Wikkit's Gate, which would ultimately destroy the galaxy.
When the robots kidnap Zaphod and Marvin the Paranoid Android, Ford joins Arthur and Trillian in rescuing them, and band together to stop the robots. they ultimately succeed in saving the galaxy, and Ford bids Arthur a fond farewell as the latter goes to vacation on a peaceful planet inhabited entirely by birds.
So Long, and Thanks for All the Fish
Months later, Ford discovers that the Guide's entry for Earth contains his original research, rather than the earlier version, "Mostly harmless". He hitches a ride on a spaceship to Earth and crashes into the middle of London, causing mass panic. He meets with Arthur and his girlfriend Fenchurch, recently returned to Earth as well, and they commandeer the ship to find Marvin, who has discovered God's final message to His creation - "We apologize for the inconvenience".
Ford returns to the Guide's main office, only to discover that it has been taken over by a Vogon-owned corporation. He steals the new revision of the Guide and goes into hiding. He fears that the Guide has become too powerful, as it now has the ability to warp and distort reality. Along with Arthur and Trillian, Ford travels to an alternate dimension in which the events of the previous books never occurred, and save Random Dent, Tricia McMillan's daughter with Arthur.
They learn that, in this dimension, the Vogons will destroy Earth with secret backing from the Grebulons, the race of beings charged with answering the question of Life, the Universe, and Everything, who have grown bored with their mission. As the Grebulons prepare to wipe out all life on Earth, Ford begins laughing. His fate, and that of all other beings on Earth, is left ambiguous, although it is implied that he and his friends may have survived.