The final entry into the radio plays shows just how far afield Adams got from his original silly little radio drama about Arthur Dent. This very abbreviated phase introduces a number of Big Ideas inspired by the original work -- the notion of a probability continuum, most notably, and "plural zones" -- and it's a shame that Adams didn't get a chance to incorporate all these ideas right from the start, as they would have certainly led to some direction in the otherwise flabby middle episodes of the series.
This phase doesn't have much urgency to the plot, and rushes through large swaths of story to reach the ending... or non-ending, as it turns out, as all the characters come back out for one final bow. This is, perhaps, as satisfying an ending as you could expect from a storyline that's taken the listener through multiple dimensions and character iterations: there isn't one pat ending, because there isn't one single storyline, but a whole great mishmash posing as a coherent story. The quintessential phase is an attempt to reign in all the scattered particles of story and put a neat bow on it. It doesn't quite work.
The neatest ending, however, would have been not to end at all and to leave the story back at the end of the original Hitchhiker's story -- Arthur and Ford, exploring a strange, unexplainable universe with only a trusty towel and a battered Guide to lead them.