Slartibartfast takes Arthur into the planet, where all the cool nightclubs are.
Actually, Slartibartfast is taking Arthur into the hyperspace dimension-place-thingy where they build planets. It's not really inside the planet, but that's how you get there.
Right now, they are building a new planet here—a second Earth, because, as Slartibartfast explains, they made the first one, too.
Slartibartfast is very proud of the fjords of Norway, which he won an award for.
He also explains that it was the mice that paid for Earth, because they were in fact hyperintelligent pandimensional beings that just look like mice in our dimension. Not just cheese-sniffing pe(s)ts. They had bought Earth as a giant computer to run a special program.
None of this makes any sense to Arthur, so Slartibartfast offers to explain.