Helen is reading a book about a girl detective named Curious Crystal to Martha. Helen wishes she were a detective, so Martha comes up with the name Heroic Helen, and decides to be her sidekick and call herself Sparky. Helen dresses up as Curious Crystal and the two start sleuthing in the kitchen.
Martha notices that tomorrow's date on the calendar is circled in red, but Helen points out that the calendar is the Lorraines' own. Then, Martha spots a trail of cobbler crumbs leading to the armchair, but then realises that she was the one who took the cobbler. They go outside, and Helen finds a small piece of paper with "The Whistler" written on one side next to a drawing of a monocle and top hat, and "you've Been Warned!!" [sic] on the other.
Then, they notice some muddy footprints, and Helen observes that it appears whoever left them was hopping. They follow the footprints (with Martha a little nervous that it might be a monster) but lose the trail because the mud wore off. However, they then hear the footsteps of a single, muddy rubber boot and hide in a bush... but it turns out to be T.D. using a muddy rubber boot on a stick to make the footprints. Martha assumes he is "the Whistler" but it turns out that he's just trying to get a mop out of O.G.'s book.
Helen unsticks it, then changes back to her normal attire due to getting mud on her cosplay outfit. At the corner store, Helen finds a note on the ground, which is a list of stores in the same handwriting as the previous note. They're all crossed off except the last one, "Flower Shop", and then a man in a suit, monocle, and top hat snatches the list. He asks for directions, but when he leaves, Helen thinks he's suspicious, since the men of Wagstaff City don't tend to dress so formally and the card depicted a monocle and top hat.
They decide to follow the man, but he's already gone. However, they realise he was headed for the flower shop, so they go there. They tell Mariella about the Whistler, and she promises to let them know if she sees a whistling man in a top hat and monocle. She then gives Helen some money and asks her to buy a chicken so she can make Danny's favourite food for dinner.
They buy one from Carl's butcher shop, where Carl reveals that the drycleaner, grocery store, and his shop have all been robbed recently. Not only that, but although he hasn't seen the robber, sources say the robber was "very well-dressed". Helen then wonders if this thief is the Whistler, since the man they saw before was indeed well-dressed.
The next day, Helen and Martha spy on the flower shop in the rain, and spot the same man dropping another note before heading off in a suspicious-looking way. Martha runs to get the police, while Helen goes to tell Mariella, and she picks up the note, which depicts sheet music for a song called "You've Been Warned". Danny walks up and asks her if "Mr. Goffin" has come up. Helen asks who that is, and he reveals that he's lost his card, which turns out to be the card Helen found. Danny reveals that it's something he's been planning as an anniversary surprise.
Meanwhile, at the police station, Martha tells the police. Back at the flower shop, Helen apologises to Mr. Goffin, the well-dressed man. Goffin walks into the flower store and confirms that the owner is Mariella Lorraine, but then the police show up with Martha. Helen stops the police from entering the store, while Mr. Goffin sings a love song to Mariella. Helen then realises that Carl was robbed after his birthday, so she wonders if all the other robbed stores got singing telegrams the day they were robbed.
Mr. Goffin is then seen opening the window, and it turns out he was indeed the robber, and he gets arrested.
This episode provides examples of
- Alliterative Title: The Crooning Crook Caper. Also, Helen's books are called Curious Crystal.
- A Birthday, Not a Break: Downplayed when Carl mentions he was robbed just after his birthday.
- Character Name and the Noun Phrase: All of Helen's Curious Crystal books are called Curious Crystal and the X.
- Chewing the Scenery: While serenading Mariella, Mr. Goffin holds his chest, lies on her desk, and does other dramatic gestures.
- Continuity Nod: Martha brings up the time she helped "put away" Louie Kablooie and Jimmy Gimme Moore (which happened in "Codename: Martha") and the crooks who were breaking into the museum (which happened in "Martha's Life in Crime").
- Damsel in Distress: In Helen's book, Crystal is tied up on a railway track.
- Fun with Homophones: Martha misunderstands "stakeout" as "steak out".
- Heroic Dog: In Helen's book, Crystal is saved from an oncoming train by her dog.
- Hold Your Hippogriffs: Martha says she's on the edge of her "tail" instead of her seat.
- Incorrect Animal Noise: Conversed when Martha thinks "bow-wow" is an unrealistic onomatapoeia.
- Kid Detective: The episode follows Helen reading some books about a little girl who does detective work, and wanting to be a detective herself. She does end up having a thief arrested.
- The Klutz: Mr. Goffin has a habit of dropping his notes, to the point where Martha calls him Professor Butterfingers.
- Mistaken for Thief: Subverted. The Whistler seemed innocent when he's revealed to be delivering singing telegrams, but he turns out to have been the robber after all.
- Mirthless Laughter: Martha chuckles awkwardly upon remembering that she was the one to steal the cobbler she was investigating the theft of.
- Mystery Episode: The episode follows Helen and Martha investigating a note that warns about someone called the Whistler.
- The Pratfall: When the mop falls out of the boot, it sends Helen landing on her behind.
- Red Herring: The muddy footprints seemed suspicious, but they turned out to be unrelated to the "Whistler" and caused by T.D. trying to unstick a mop from a boot. Helen even calls this a red herring.
- Spell My Name with a "The": Helen and Martha are searching for someone called the Whistler or the Gentleman.
- Sticky Fingers: Mr. Goffin stole from several unrelated stores, and he's clearly not poor based on his clothes, so he's either greedy or just likes to steal.
- Stock Animal Diet: Martha gets excited when she thinks "stakeout" is "steak out", then she eats a sausage at Carl's butcher shop.
- You Can Talk?: Upon hearing Martha talk, the suited-up man says, "A talking dog? How curious!".
