A writer takes a job as a secretary because her scrupulous husband isn't bringing in the dough as an attorney. When her new employer is murdered, she can't seem to make up her mind as to whether she "dunnit" or not.

There are plenty of guilty secrets at the school where Hildegarde Withers teaches. When she finds the body of the pretty music teacher, she calls in her old friend Inspector Piper, who promptly arrests the obvious suspect. Clues multiply and everyone looks suspicious as Piper and Miss Withers continue their battle of the sexes.

A wealthy society doctor decides to research the medical aspects of criminal behaviour by becoming one himself. He joins a gang of thieves and proceeds to wrest leadership of the gang away from its extremely resentful leader.

A mad scientist attempts to rule the world by creating various elaborate inventions.

Ellery Queen solves a mystery involving a valuable stamp.

Arthur T. Ryan, a hitchhiker, gets a ride from haughty, society girl Glenda Wynant and her fiance, wealthy J. F. Van Avery after he helps them to replace the top of their convertible when it begins to rain. As they approach a bridge, Art notices a few stalled cars, and when the storm worsens, the bridge washes away, leaving Art, Glenda, Van and several others stranded in a canyon.

The Progressive Party convention is deadlocked for governor, so both sides nominate the dark horse Zachary Hicks. Kay Russell suggests they hire Hal Blake as campaign manager; but first they have to get him out of jail for not paying alimony. Blake organizes the office and coaches Hicks to answer every question by pausing and then saying, "Well yes, but then again no." Blake will sell Hicks as dumb but honest. Russell refuses to marry Blake, while Joe keeps people away from Blake's office. Blake teaches Hicks a speech by Lincoln. At the debate when the conservative candidate Underwood recites the same speech, Blake exposes him as a plagiarist. Hicks is presented for photo opportunities and gives his yes-and-no answer to any question, including whether he expects to win.

Socialite Pat Reynolds (Ida Lupino) is forced to become the "smart girl" of the title when her wealthy father commits suicide, leaving nothing but a pile of debts. Pat sets up a successful hat-designing business, providing the sole support for herself and her sister Kay (Gail Patrick). So devoted is Pat to Kay's welfare that she stands by in stoic silence as Kay begins romancing Pat's sweetie Nick Graham (Kent Taylor).

A decoding expert tangles with enemy spies.

The title character is a resourceful young man who knows a whole little about a whole lot of things, and who concentrates by playing his saxophone. Clarence ingratiates himself with the wealthy and eccentric Wheeler family, though daughter Cora can't stand the boy.