All the departing Dolphins leave Earth, knowing its imminent destruction.
One Thursday around lunchtime, Arthur Dent discovers that his house is to be immediately demolished in order to make way for a bypass. He tries delaying the bulldozers by lying down in front of them. Ford Prefect, a friend of Arthur's, gives all the workers alcohol to stop them demolishing his house. He convinces Arthur to go to the pub with him. Over six pints of beer for "muscle relaxant" and some packets of peanuts for the salt, Ford explains that he is an alien from a planet in the vicinity of Betelgeuse, and a journalist working on the Hitchhiker's Guide to the Galaxy, a universal guide book, and that the Earth is to be demolished later that day by a race called Vogons, in order to make way for a hyperspace bypass.
The duo discuss Tricia McMillan, a girl Arthur had met at a fancy dress party while dressed as Henry Morton Stanley. Tricia was dressed as Charles Darwin, but everyone thought she was Santa Claus. She wanted Arthur to come with her to Madagascar but instead she went away with Zaphod Beeblebrox, an alien "from another planet".
By this time, Arthur's house has been demolished but all the workers flee at the appearance of a large Vogon ship. Prostetnic Vogon Jeltz announces the plans for the demolition of Earth had been available at Alpha Centauri for fifty Earth years. He then gives the instruction to commence the demolition. Ford saves himself and Arthur by hitching a ride on a Vogon ship. The two are discovered in a washroom of the ship and taken to hear a recital of some Vogon poetry. After trying to compliment the poem, they are thrown out of an airlock. Their chance of surviving was two to the power of 2079460347 to one against.
They miraculously are picked up by the star-ship Heart of Gold. Marvin the Paranoid Android collects them from Entry Bay 2. They find Ford's "semi-half brother" Zaphod Beeblebrox, the President of the Galaxy. He has stolen the ship along with Tricia McMillan, who has changed her name to Trilian because "it's a bit more spacey". Arthur confronts Trillian about why she left him for Zaphod and she explains that she wanted to experience life and travelling around the Universe is one way to do that.
After having a Pan Galactic Gargle Blaster with Ford, Zaphod shows Ford and Arthur a recording of the supercomputer Deep Thought and how it designed Earth to determine the Ultimate Question of Life, the Universe and Everything. Zaphod explains that he is seeking the planet Magrathea, where he believes he can discover the Question.
However after using the Infinite Improbability Drive in an attempt to reach Magrathea, they instead end up orbitting around the planet Viltvodle VI. Zaphod decides to visit Humma Kavula, his opponent from the election, to settle a score. Upon learning of Zaphod's plan, Kavula announces that he has the coordinates to Magrathea. He takes one of Zaphod's two heads hostage and demands they bring him the Point-of-view gun created by Deep Thought, which allows the target to understand the shooter's point of view. As they are leaving the planet, Trillian is captured by Vogons. The others travel to rescue her from the Vogon home world bureaucracy, facing long lines and frustrating form processing. Trillian is outraged to learn that Zaphod signed the authorization for the destruction of Earth thinking it was a request for an autograph. The Heart of Gold is chased by the Vogons, led by Galactic Vice-President Questular Rontok, who is attempting to rescue Zaphod from himself. As the Heart of Gold arrives in orbit above Magrathea, Arthur triggers the improbability drive to avoid the automated missile defense systems. The missiles transform into a bowl of petunias and a sperm whale. On the planet, Zaphod, Ford, and Trillian take a portal to Deep Thought. When they ask the computer whether it has calculated the ultimate question, it reveals that it designed another supercomputer to do so—Earth. When the trio finds the Point-of-View gun, Trillian shoots Zaphod, making him understand how she feels about the destruction of Earth. She also finds out how much she loves Arthur. Arthur and Marvin miss the portal and encounter a Magrathean called Slartibartfast, who takes Arthur on a tour of the construction floor where Earth Mark II is being built. Slartibartfast takes Arthur home, where the others are enjoying a feast provided by pan-dimensional beings who resemble a pair of mice. Arthur realizes he has fallen into a trap. The mice, who constructed Deep Thought, used the supercomputer to build an even larger supercomputer, the planet Earth, to determine the Ultimate Question. Believing Arthur, the last remaining supercomputer component, may hold the Ultimate Answer, the mice attempt to remove his brain. Arthur kills the mice. As the crew regroup outside the house they are surrounded by Vogons and take shelter in a caravan as the Vogons open fire. Marvin is left outside and shot in the back of the head, and uses the Point-of-View gun on the Vogons, causing them to become depressed and unable to fight. As the Vogons are taken away and Questular rejoins with Zaphod, Arthur chooses to explore the galaxy with Trillian and lets Slartibartfast finalize the new Earth without him. The Heart of Gold crew decide to visit the Restaurant at the End of the Universe while Marvin points out they are going the wrong way.
- Martin Freeman as Arthur Dent
- Sam Rockwell as Zaphod Beeblebrox
- Mos Def as Ford Prefect
- Zooey Deschanel as Tricia McMillan / Trillian
- Bill Nighy as Slartibartfast
- Warwick Davis as Marvin the Paranoid Android (voiced by Alan Rickman)
- Anna Chancellor as Questular Rontok
- John Malkovich as Humma Kavula
- Kelly Macdonald as Jin Jenz Reporter
- Jason Schwartzman (uncredited) as Gag Halfrunt
- Edgar Wright (uncredited) as Deep Thought Tech
- Simon Jones (cameo) as Magrathea Video Recording
- Stephen Fry as Narrator, The Guide
- Alan Rickman as Marvin the Paranoid Android
- Helen Mirren as Deep Thought
- Richard Griffiths as Jeltz
- Thomas Lennon as Eddie the Computer
- Bill Bailey as The Whale
- Mak Wilson as Vogon Interpreter
- Garth Jennings as Frankie Mouse (uncredited)
Official Site The official Hitchhiker's Guide to the Galaxy film site.