We zoom out to see that alien ships are coming to Earth. For a picnic? Unfortunately, no.
Only Ford has a "Sub-Etha Sens-O-Matic" device that tells him when aliens are coming, so only Ford is prepared for the destruction of Earth. Has he taken out insurance on Earth? No, but he's got his towel.
This leads to one of the most important digressions in this very digressive book. The whole digression is all about how important towels are when one is hitchhiking across the galaxy. That's why, in the real world, May 25th is a memorial to Douglas Adams called "Towel Day".
Back on Earth, Prosser destroys Arthur's house. This makes Arthur threaten to kill all of them—or as the British say, this makes Arthur rather cross.
Also, while no one at the pub believes Ford about the destruction of Earth, the barman gets an eerie feeling and asks if there's anything that can be done. Can they heroically fight off the aliens like inIndependence Day? Or make friends with the aliens like in E.T.? No, they cannot, says Ford.
But Ford does ask the barman for some peanuts, so the barman isn't totally useless.
Around this time, the alien ships show up and freak everyone out, probably because the ships are yellow. Or, as they say in England, yellow.
Only Ford isn't freaking out about the Vogon aliens, because he knows where his towel is.
The Vogons alert the people on Earth that their planet has to be destroyed to make way for a "hyperspatial express route." Hey, that's pretty much why Arthur's house had to be torn down, right?
In fact, the leader of the Vogons, Prostetnic Vogon Jeltz, uses the same arguments that Prosser used: you should have known about these plans, no use complaining now, the march of progress, blah blah blah.
And then the Vogons destroy Earth. A typical Thursday, really.