After a very unusual night, Richard becomes re-acquainted with his college friend, Svlad Cjelli - or, as he is currently calling himself, Dirk Gently. There's also a ghost involved. It gets weird. 
 I have my doubts that Adams knew what the solution to the mystery would be before he started writing. This was my impression with the Hitchhiker's books as well - he seems to just sit down, write what's funny, and then try to come up with something that'll end the book. 
 And that's fine. This is one mystery where the journey really is all that matters, and the journey is hilarious. 
 Now that I've finished reading the book, I can finally watch that show I keep hearing about!