The first episode (of The Tertiary Phase) finds us on prehistoric Earth with Arthur. The quick and dirty version of what occurs next: The narrator fills in some of the back story. Arthur Dent's encounter with Wowbagger the Infinitely Prolonged and the narrator's description thereof. Arthur's reunion with Ford Prefect and Ford's discovery of a hole in the spacetime wash (the chesterfield sofa). Their escape from prehistoric Earth. The narrator on Starship Titanic and the creation of the Heart of Gold. Marvin and the mattresses. Trillian and Zaphod Beeblebrox on the Heart of Gold. Trillian's escape from Zaphod.
For the record, I have no qualms revealing the above. Much of it (on the order of > 80%) parallels the books. In addition, it's not the details of the story that form the basis for the radio show but rather the language used and method of telling. Which, incidentally, are spot on. BBC got this one right. The new episodes feature about half of the originally cast which is terrific. Arthur, Zaphod and Marvin all have the same voice actors. The others are different but not terribly so.
In all, the episode works. I wish there was less explaining of back story and more of the new story. But there will be more episodes and, if the BBC Radio 4's claims of Quadrenary and Quintessential Phases is anything to go by, there will be LOTS of new episodes. If it weren't for the fact that the episode is extremely well done and completely in the style of the existing, earlier episodes, I could see fans getting upset. However, given the aforementioned aspects, I hope fans don't get too riled up. It sounds good! It sounds good!!! So relax, enjoy and become enchanted (again).
ALAN'S RADIO REVIEW: LISTEN TO THESE OR ELSE*!!!!
*That is, or else suffer a horrendous fate. (e.g. Being fed to the Ravenous Bugblatter Beast of Traal.)
ADDENDUM: Whoops! I forgot that I have three quotes from the episode.
"I have detected disturbances in the wash."
"Insanity is a gradual process, don't rush it."
"I'm not going to ask but imagine I have."