Arthur Dent has woken up with a hangover on his least favourite day of the week, but this day is about to get worse still.  Outside his front door is a bulldozer preparing to demolish his house to make way for a new bypass.

Arthur may think the day cannot possibly get any worse, but his peculiar friend Ford Prefect has even more dire news: the entire Earth is about to be demolished by a Vogon Constructor Fleet currently in orbit around the planet. Why? To make way for a new hyperspace bypass of course.

Does this mean Arthur and his really quite interesting career in local radio are doomed? Not a bit of it, because Ford Prefect is not an earthling at all but an alien and a roving writer for The Hitchhiker's Guide to the Galaxy. The two friends hitch a lift aboard one of the Vogon ships just before the Earth is destroyed.  But their luck doesn’t improve: they are cruelly tortured with truly terrible poetry before being expelled into the empty, airless wastes of space.  Surely this time they must perish?

No again, as they are most improbably picked up by another spaceship piloted by the President of the Galaxy himself, the one and only Zaphod Beeblebrox, and his companion Trillian. Arthur and Ford join Zaphod, Trillian and their depressed robot Marvin on their quest to find Magrathea, a legendary lost planet. 

On Magrathea, Arthur discovers that planet Earth was in fact a vast super-computer designed to elucidate the great question of life, the universe and everything.  By a curious series of events, the race of pan-dimensional beings who commissioned the computer already have the answer to life, the universe and everything – but they’ve found it’s not much use without the question.

The Earth’s calculations were due to run for ten million years, but just five minutes before completion the Vogons destroyed it.  Arthur’s survival offers the pan-dimensional beings an alternative to beginning again with an Earth mark II, if Arthur would just consent to let them extract his brain.

Arthur is understandably not keen on the idea and he, along with Zaphod, Ford, Trillian and Marvin, depart Magrathea in rather a hurry, bound for a late lunch at the Restaurant at the End of the Universe.