So what, I wonder, could compel Dorothy to become a detective? Well, a visit to Munchkinland starts things off as the statue of Glinda is missing her wand. The Munchkins think it was stolen, but Dorothy uses a little logic and her knowledge of the previous night's weather to find it had blown into the duck pond. So the Munchkins ask her about a whole bunch of other mysteries, which she manages to solve pretty rapidly. Dorothy then tells the Scarecrow, Tin Woodman, and Cowardly Lion about all the detective movies she saw back in Kansas with Uncle Henry, notably ones featuring Sam Spade ("Isn't he the talking shovel who lives just outside the Emerald City?" asks the Tin Man) and Phillip Marlowe. By the time they get back to the Emerald City, a line has formed for Dorothy's services. So, she goes into business, with the proper setting. She sets up a darkened office, puts on a fedora and trench coat, and starts narrating a montage of her cases. It ends with the Scarecrow turning on the light and asking, "Who are you talking to?" A big bang just outside the city gates brings them all outside to find Ozma and the Wizard in the vicinity. Ozma has no recollection of what caused the bang, so it's up to the Wizard to give an account of what happened. It seems a blur attacked Ozma. Dorothy follows the clues, and finds Wilhelmina hiding in the roadside rosebushes. It looks bad for her, but she protests her innocence. This being Wilhelmina, nobody quite believes her, but Dorothy thinks it's all a little too neat, and the Wizard seems especially eager to put her into custody. Sure enough, another examination of the clues unveils new evidence, and Dorothy is able to prove Wilhelmina's innocence, even at the expense of uncovering the young witch's love of roses. No, it was a charm in the Wizard's back pocket that went off and caused the chaos. Knowing he messed up, he decided to cover his tracks by putting the blame on Wilhelmina after spotting her in the rosebushes. The case is solved, but Wilhelmina is not at all happy with the Wizard and vows revenge. Dorothy decides Oz has a lot of mysteries that don't all need to be solved, so she shuts down the agency.
Yeah, this was all very silly, and just a way to tell an old-fashioned hard-boiled film noir-style detective story. But they have fun with it. The Tin Man, especially, has no idea what Dorothy is talking about half the time, and the boys have some fun popping holes in the tropes of the genre. Detective Dorothy is cute, but I'm glad his is only a one-off, and she's not going to keep it. Finally, now that they've opened that bag, I want to see Sam Spade the talking shovel appear in the show!