If this plot was making too much sense and moving too fast for you, you can take a small breather here, since we get a digression.
Arthur's comment about how he was having difficulty with his lifestyle fell through a wormhole in space-time. It landed at a peace meeting between two species that were about to go to war (or make peace). But just at that moment, Arthur's comment slipped in, and it was a huge insult in one of their languages.
After thousands of years of war, these two species made peace and realized that the terrible insult actually came from our galaxy. So they sent a giant war fleet to destroy Earth.
Unfortunately for them, they miscalculated the size of Earth, and the whole fleet was swallowed by a small dog.
We hope you enjoyed that digression, because we return now to the main story.
Slartibartfast brings Arthur to a room where Zaphod, Ford, and Trillian are already eating a big meal.
Zaphod, Ford, and Trillian introduce Arthur to their hosts, Benjy mouse and Frankie mouse, who were the white mice that Trillian brought from Earth. (So, in Chapter 19, when the ship let out two groups of people, these mice were the second group.)
Benjy mouse and Frankie mouse (no relation) tell Slartibartfast that they won't need a new Earth after all, which upsets him. Because, of course, he was really looking forward to making more fjords.
The mice have a proposition for Arthur and his friends: since Arthur was on Earth right before it went ka-blooey, his brain may hold the question that the computer was trying to find out.
Frankie and Benjy need to know that question because they've got some great offers for talk show appearances (and probably book deals) back in their own dimension.
All Frankie and Benjy want is to buy Arthur's brain, chop it up, find out the question that's hidden there, and then go off to become ridiculously famous and wealthy in their own dimension.
Frankie and Benjy say they'll totally replace Arthur's brain with a simple electronic brain, so no one will notice. As Zaphod points out, all this new brain would need to say is "What?," "I don't understand," and "Where's the tea?" (89). That seems pretty accurate to us.
Arthur doesn't want to have his brain chopped up for science, but the mice won't take no for an answer.
The mice's thugs get ready to attack when suddenly all the alarms on the planet go off. It's probably just someone burning the popcorn again, setting off the fire alarm, right?