As a continuation of Douglas Adams' famous The Hitchiker's Guide Series
this was, as indicated by the foreword, one of the most plotted in the series. But as also indicated by the foreword, you don't read The Hitchiker's Guide Series
for the plots. So, you ask me, what do you read it for? You read it for the sense of wonder about the crazy place the universe is. You read it for the comedy of Douglas Adams, for his creative and zany use of made up people, places, words...for his use of language. He is a wizard, transforming words into wit to power a laugh within the inner sanctum of your mind as a reader. When you think you've got him figured out, that's when you realise that actually you haven't.
I read elsewhere when attempting to discover what I could about the literary idea of 'deus-ex-machina' that while it is generally frowned upon as poor storytelling that Adams was able to use it brilliantly for humour. Reading this third instalment of his series I saw again that yes, he was able to do exactly that! And at the same time his use of deus-ex-machina also contributes ultimately to the plot (which we as readers of Adams do not care for). In many ways, perhaps unintentionally, Adams therefore shows that he can also use the literary device of 'Chekhov's gun'. Characters and plot ideas introduced earlier in the piece never really go away. Some may be simple ideas thrown in their for an occasional laugh, but if you see Adams mention a fact or a character specifically, especially in a way that's out of the story's usual context then that character or fact will appear later. Such as the idea in this story of flying (and the re-incarnated character - which I thought was brilliant!).
I won't bother with a plot summary. I doubt anyone can sum up the plot in any way that makes much sense. I will say that if you've read the previous books and enjoyed them then this is a similar continuation. If you haven't read any of the previous books don't jump in now. I recommend going back to where there's Vogon poetry and the destruction of the world with The Hitchhiker's Guide to the Galaxy.