The story is a prequel to the events in The Hitchhiker's Guide to the Galaxy and has the young Zaphod Beeblebrox working as a salvage ship operator. He guides some bureaucrats to a crashed spaceship that may be leaking some dangerous materials, radioactive, toxic and otherwise hazardous by-products which were destined to be thrown into a black hole. The bureaucrats swear that it is "perfectly safe." When asked why they want to see it if that is true, they claim that they "like looking at things that are perfectly safe." The comic asides in the story include some of the time travel paradoxes which are a common running theme in Adams' SF work, and plenty of material about lobsters.
Since this was before Zaphod blocked off sections of his own brain for the presidency, readers are able to glimpse what his original personality was like. His general speech patterns and goof-off personality are the same, but he seems to have moral views and is more likely to go off on life-threatening and exciting quests for the greater good.
Throughout the story, it is emphasised that there is something particularly dangerous on board that ought to have been utterly destroyed, but is feared to have escaped.
Ultimately, it is revealed that the something was actually three identical "Designer People". The personalities seem totally benign, which is what makes them so dangerous. The ship is filled with substances so dangerous that they are safe because no one who would actually use them would be let near. The personalities, products of a Sirius Cybernetics Corporation project, however, have custom personalities that could not naturally exist. There is "nothing they will not do if allowed, and there is nothing they will not be allowed to do." Since no one will recognise that they are capable of mass destruction (despite their good intentions), no one will stop them from doing the unspeakable.
The story culminates with the revelation that one of the personalities has escaped and headed off into Galactic Sector ZZ9 Plural Z Alpha, which is where Arthur and Ford Prefect were picked up by the Heart of Gold a fraction of second before they would have perished (and just minutes after they had been rescued from the demolished Earth in the same sector), in the original Hitchhiker's Guide to the Galaxy.
Though it does not state it specifically, the story implies that this unspeakably dangerous creation is now known to the planet Earth as then-President Ronald Reagan, a reflection of the author's critical views about the policies of then-President Reagan which also surface in Mostly Harmless. A version of the story included in the posthumously-published The Salmon of Doubt makes this explicit.
This link below should be the full story in pdf format if it works.
http://lib.ru/ADAMS/dayz1_.txt