“The Case of the Sleepwalker’s Niece” is another fascinating journey into Perry Mason’s world. Gardner actually ends the book with a shout-out to his next Perry Mason novel, “The Case of the Stuttering Bishop,” indicating it was already ready to roll. At the start of this novel, Mason spars with Della Street about what case he should take, noting that a lawyer has to wade through many uninteresting murders to get one that is exciting and that dead men are never interesting, only the live ones count. He wants to take a case about a sleepwalker because his keen legal mind can appreciate that, if really true, there would be no malice and no premeditation. Mason once again nearly gets caught one time making out with confidential secretary Della Street, who seems to not want Mason to take the niece’s case because the niece is pretty with a “stunning figure.” Mason though quips that, if a pretty 23 or 24 year old girl “with a swell figure can’t cross her knees in the witness box and convince a jury her uncle’s a sleepwalker, I’ll quit trial work.”
Edna Hammer, the niece, tells Mason that her uncle, Peter B. Kent, an eccentric millionaire, has a sleepwalking problem. She also says he is married to a “hell-cat” and that he is going through a divorce, but now the golddigger is trying to bleed him try and wants to call off the divorce, putting Kent in a vice because he is already engaged to the next Mrs. Kent, a nurse. Also, when sleepwalking, he took a carving knife and put it under his pillow and the wife, Dora, thinks he was trying to kill her by faking the sleepwalking.
Edna has convinced her uncle through astrological signs and the like that he should consult Mason, but wants the uncle to believe it was his own idea and slips out the back door. The uncle tells Mason he has the divorce problem and also that a business partner from Chicago is now threatening to sue him for millions about a patent issue.
Somehow, everyone gathers under the Kent roof in his mansion, Edna, her boyfriend, the business partner, the business partner’s attorney, Mason, and the betrothed nurse. Mason has an idea that he can send his junior associate Jackson to Santa Barbara to finalize the California divorce and simultaneously send Kent and the nurse to Yuma, Arizona, to quickly get married before Doris realizes what is going on. They are sent away early the next morning before Mason and Edna together discover a bloody carving knife that was supposed to be locked in the sideboard under Kent’s pillow and Kent’s half-brother Rease dead from knife wounds in a bedroom that was supposed to be the hostile business partner’s bedroom.
When the business partner’s attorney claims he saw Kent in a white nightgown in the yard with a knife (sounds like the board game Clue, doesn’t it), Mason has his hands full because, to all purposes, it appears that his client had motive, opportunity, and intent to murder his business partner with the half-sibling getting the knife instead. No one really seems to believe that the sleepwalking defense will work with the sideboard having been unlocked and the bloody knife under the pillow and the intent to kill the business partner all to easy to see.
There are a lot of complicated moving parts at work here and even Mason does not quite figure it all out until near the end of the trial. Mason has a few tricks up his sleeve, but does not really skirt the edges of the law in this novel.