Jeltz announces that he's going to find and kill the hitchhikers by throwing them out an airlock. But first they're going to jump through hyperspace to a nearby star. Ford tells Arthur that going into hyperspace is like being drunk, which isn't fun—not for the liquid being drunk, at least.
Arthur asks Ford about the fish in his ear, which means we're about to get a lesson from the Guide.
The Babel fish (says the Guide) is the weirdest fish ever: it feeds on the brain energy of people around the host and excretes (ew) brain waves into the host. Which basically means that it translates anything you hear into language you can understand.
Some people think the Babel fish proves that God doesn't exist by proving that God does. Wait, what? The idea goes something like this:
Proof denies faith, says God, and faith is necessary. But, says Man, the Babel fish couldn't evolve naturally and clearly was created by God. Therefore the Babel fish proves You exist—which means that You don't. We find this joke hilarious, but if you don't, the secret is to keep reading it over and over again until you do.
Also, there's a cultural translation problem here: when the narrator says that Man got killed at the next "zebra crossing," that's just British-talk for a crosswalk for people—not a crosswalk for zebras. Though that's a pretty funny image.
After the trip through hyperspace, Arthur tries to deal with the whole "my planet exploded" feeling. It doesn't go well.
Arthur asks to see the Guide's entry for Earth and is disappointed to see that it only says "Harmless."
Ford tries to explain that there are lots of planets out there and not a lot of space in the book. And on the bright side, Ford helps to extend the entry on Earth so that it reads "Mostly harmless."
Arthur freaks out, but luckily, the Vogons come to kill them at this time.