He would find where his cave had been.
On prehistoric Earth he had lived in a cave, not a nice cave, a lousy cave, but ... There was no but. It had been a totally lousy cave and he had hated it. But he had lived in it for five years which made it home of some kind, and a person likes to keep track of his homes. Arthur Dent was such a person and so he went to Exeter to buy a computer.
That was what he really wanted, of course, a computer. But he felt he ought to have some serious purpose in mind before he simply went and lashed out a lot of readies on what people might otherwise mistake as being just a thing to play with. So that was his serious purpose. To pinpoint the exact location of a cave on prehistoric Earth. He explained this to the man in the shop.
``Why?'' said the man in the shop.
This was a tricky one.
``OK, skip that,'' said the man in the shop. ``How?''
``Well, I was hoping you could help me with that.''
The man sighed and his shoulders dropped.
``Have you much experience of computers?''
Arthur wondered whether to mention Eddie the shipboard computer on the Heart of Gold, who could have done the job in a second, or Deep Thought, or --- but decided he wouldn't.
``No,'' he said.
``Looks like a fun afternoon,'' said the man in the shop, but he said it only to himself.
Arthur bought the Apple anyway. Over a few days he also acquired some astronomical software, plotted the movements of stars, drew rough little diagrams of how he seemed to remember the stars to have been in the sky when he looked up out of his cave at night, and worked away busily at it for weeks, cheerfully putting off the conclusion he knew he would inevitably have to come to, which was that the whole project was completely ludicrous.
Rough drawings from memory were futile. He didn't even know how long it had been, beyond Ford Prefect's rough guess at the time that it was ``a couple of million years'' and he simply didn't have the maths.
Still, in the end he worked out a method which would at least produce a result. He decided not to mind the fact that with the extraordinary jumble of rules of thumb, wild approximations and arcane guesswork he was using he would be lucky to hit the right galaxy, he just went ahead and got a result.
He would call it the right result. Who would know?
As it happened, through the myriad and unfathomable chances of fate, he got it exactly right, though he of course would never know that. He just went up to London and knocked on the appropriate door.
``Oh. I thought you were going to phone me first.''
Arthur gaped in astonishment.
``You can only come in for a few minutes,'' said Fenchurch. ``I'm just going out.''