Once you become an amateur detective, it becomes traditional for you to pretty much stumble over murder victims on a regular basis.
Hildegarde is keeping a student after school for punishment when she discovers the music teacher dead--someone applied a blunt instrument to her head. The equally acerbic Inspector Oscar Piper is once again called to investigate.
But the body has disappeared. It's actually an important bit of character interaction that, though Piper is at first annoyed to find there is no body, he doesn't really doubt Hildegarde at all. The two might still trade some pretty sharp barbs, but after the events of The Penguin Pool Murder, they know they can trust each other.
(In fact, at the end of the first movie, they were rushing off to get married. Blackboard doesn't explain why they aren't married now--though the original novel does: Piper was called away on another case before they got hitched, giving Hildegarde time to change her mind.)
From there, the movie adopts an interesting structure. Hildegarde is certain the murderer hasn't had a chance to leave the school building, so the first half-hour of this 71-minute film involving searching the school for both a corpse and a killer. They find the corpse in the school's furnace, but the killer manages to slug a cop and give them the slip.
From there, it's a traditional mystery--questioning suspects and searching for clues. There's a wonderful scene in which Piper and Hildegarde return to the school and search the cellar, only to have a mysterious assailant throw a hatchet at Hildegarde's head.
Several small clues--including musical notes written on a blackboard and a dead ant in a glass beaker--point Hildegard to the killer. But to get real proof, she and Piper will have to set a trap.
Like the early film, Murder on the Blackboard is a reasonable mystery turned into a delightful time by Edna May Oliver as Hildegard and James Gleason as Piper. The two continue to play off each other perfectly, getting on each others nerves but still working in tandem to solve a crime. Hildegarde Withers is a part Edna May Oliver was born to play, while Gleason was her perfect foil.
There's an interesting bit of trivia concerning the supporting casts of these first two films, by the way. This series was produced by RKO, which also made King Kong in 1933. The Penguin Pool Murder was made in 1932 with Robert Armstrong in the cast. A year later, Armstrong and Bruce Cabot were running around Skull Island avoiding dinosaurs and giant apes. Then, a year after that, Bruce Cabot had his turn playing a role in a Hildegard Withers movie--he's a science teacher in Murder on the Blackboard.
Edna May Oliver will play Hildegarde Withers one more time, so that will be the next film we look at in this series. There were several more Hildegarde films starring other actresses. These are quite good, but in the end Edna May is the only true Hildegarde Withers.