Malevolent American financier Sigsbee Manderson is dead, and the world's financial markets are sent into chaos at the news. Amateur detective and newspaper reporter Philip Trent arrives at Manderson's estate to look into the matter. Trent learns from Nathaniel Cupples, an old friend who is also Mrs. Mabel Manderson's uncle, that Sigsbee Manderson was detested by almost all who knew him, and in particular by his British secretary John Marlowe, his American secretary Calvin Bunner, and his new widow, Mabel, who exults at his death.
Trent observes a number of peculiarities about Manderson's body and seeks clues left on clothing and objects in the room. When Trent suggests that it might have been an affair between Mabel and Marlowe that contributed to Manderson's demise, Mabel reacts poorly, and Trent subsequently withdraws from the case. A year later, however, he meets Mabel Manderson again. Realizing he has fallen in love with her, he proposes marriage, and she accepts. Trent reopens the case, with results that have astonished readers ever since nearly as much as the detective himself. When the solution to the mystery is finally revealed, not by his own painstaking investigations, but by the genial, apparently inconsequential Cupples, Trent vows that the Manderson murder will be his last case.
An American multimillionaire, a power in the world's finance, is murdered on his estate on the south coast of England. Half a dozen persons are presented to the reader as possible objects of suspicion — the dead man's young wife, his American secretary, his English secretary, an elderly Englishman with whom he has had a violent quarrel, his butler, and a French maid. Trent, a painter, who on several previous occasions has shown decided talent in solving criminal mysteries, is sent to the scene of the crime by a great London newspaper.
There is the inevitable foil in the person of Inspector Murch, of the official police, whose years of experience in the practical service of Scotland Yard avail him but little when pitted against the superior imagination of the brilliant amateur. Trent finds the key to a greater part of the mystery in a pair of worn patent leather shoes that had belonged to the dead American multi-millionaire, and in certain fingerprints. But the story of the affair that he writes out but does not send to his newspaper lacks accuracy in one or two important points, the explanation that seems to cover everything when the book has run less than two-thirds its course is not quite complete, and it is not until the final chapter is reached that the reader is in possession of the full account of the events surrounding the death of Sigsbee Manderson.