Ronald Mason wants to join the Baker Street Irregulars, but in order to do so has to write an article on some point regarding the Sherlock Holmes stories. One of the Sherlock Holmes stories mentions a book, The Dynamics of an Asteroid, as having been authored by Professor Moriarty, and the task Mason has set himself is to determine exactly what that book is about—and he does, with help from the Black Widowers.
This story has a germ of truth in it, in that Asimov himself wanted to join the Baker Street Irregulars (or, more accurately, legitimize his membership) by the writing of such an article, and here he wraps that article in a fictional coating and presents it to the mystery reader. Personally, I find his solution a little far-fetched and the story generally weak and uninteresting, largely because the reasoning behing the conclusion is decidedly tenuous, although the conclusion itself (that the asteroid in question is one which exploded to form the current asteroid belt, and that Moriarty was interested in the problem because the idea of blowing up an entire planet appealed to him immensely) is a neat one.
