The super short version of The Hitchhiker's Guide to the Galaxy is, well, weird stuff happens to Arthur Dent, regular Earth-person. But the real joy of this book is in the weird stuff; and there's so much of it that we can't even capture it all.
The longer short version starts when Arthur's house gets torn down so a road can be built—and his day goes downhill from there. His friend Ford Prefect is an alien. That's actually the good news of the day, because it's Ford who can save both of them when the alien Vogons come to destroy the Earth to make way for some sort of space road. Ford and Arthur hitch a ride with the Vogon ship's cooks. Unfortunately, the Vogons don't like hitchhikers, so Arthur and Ford get thrown out of the spaceship to die in the cold void of interstellar space, where not even Starbucks exists.
Meanwhile, Ford's semi-cousin Zaphod Beeblebrox and his human companion Trillian steal the Heart of Gold spaceship, which has an amazing engine that can do all sorts of improbable things, such as get through LA without dealing with traffic. Also on the spaceship is a very depressed robot named Marvin. When Zaphod and Marvin use the Improbability Drive, they end up picking up Ford and Arthur on their way to the mythical planet of Magrathea.
Magrathea was a planet that made other planets for rich people (the cosmic 1%), but it disappeared a while ago. Zaphod isn't even totally sure why he wants to find it, but that's because his brain doesn't work properly. When he does find the planet, it seems dead—but it isn't. Zaphod, Ford, and Trillian go exploring inside the planet, while Arthur and Marvin are left on the surface. It's then that this dude from Magrathea named Slartibartfast finds Arthur and tells him the truth about Earth.
So, get this: Earth was actually a giant computer created to find the question that would give meaning to life. The people who ordered Earth already have the answer to the meaning of life, and that answer is: 42. The only trouble is that they're not sure what the question is that "42" answers. But they're pretty sure they can get it by chopping up Arthur's brain, since he was a part of the Earth-computer, and he was there at the end.
Arthur and his friends escape—even though the cops are after them after Zaphod stole the Heart of Gold—and decide to go get a bite to eat.