The one question that's plagued Taran all his life is just who is he? Who is his family? Is there any chance at all that his family is some sort of nobility? This last question has taken on a new level of importance to him as he has realized that he's in love with a princess and can't do anything about it without that nobility.
Dallben can't answer the question for him -- but he allows Taran leave to go try to find the answer himself. I've never understood just how Taran can pull this off -- there's practically no birth records in Prydain (I can't imagine), it's not like he can get blood tests done -- and he doesn't really interview anyone, just meanders around.
Still, he visits various corners of the kingdom -- visiting friends old and new, dipping his toe in all sorts of trades and vocations. He renders aid, and gets aid. Fflewddur Fflam shows up and spends a good portion of the novel traveling with him (Gurgi remains a constant companion). There's a confrontation with a wizard, a regional armed conflict to try to settle, a mercenary band to deal with -- as well as other woes.
He learns a lot, he matures a lot, and maybe even gets a dose of wisdom. It's not your traditional fantasy novel by any sense, but it's a good one.
As for the audiobook? Everything I've said about the other books in the series -- Alexander's introduction and Langton's performance -- holds true for this one.
The most emotionally rich of the books, the most thoughtful -- particularly for those of the target age. Good, good stuff.