Martha and T.D. appear as detectives, but then it turns out to be all in their imagination and T.D. is asked to do the dishes. While he does the dishes, he tells Martha about yesterday's "case": it started on the bus, which Danny was driving, and he (T.D.) was bored, so he decided to ride the bus to the end of the line. He got off upon seeing a sign advertising free potatoes shaped like scientists' heads, but when he opened the box, he found none. Instead was a toy dinosaur.
Back in the present, T.D. tells Martha something he considers even weirder: he'd lost a similar toy dinosaur last month, as part of an "experiment" to launch it into space. In the present, Martha wonders if it's the same one, but T.D. points out that the rocket didn't leave any marks. He then notices there is something inside the dinosaur, so he opens it up and it turns out to be a piece of paper. It appears to be a grocery list, but T.D. thinks it's a secret code because it lists more hot dogs than buns. Then, he sees some more writing on the back: "J372.5G".
T.D. looks the phrase up in many codebreaking books, but can't find it, but then Martha realises the paper smells of library books. They realise the number isn't a code, but rather a library number, so they find the book the page was torn from. However, the book has the innocuous title Funny Games for Boys and Girls. Martha gets the idea to find who took the book out last, but the librarian claims it wasn't checked out for over fifty years.
T.D. looks through the book and a note falls out that reads, "Stinky, let's rendezvous at my aunt's house after school. 13 Baker Street — Rufus." They go to the address, only to find no building there. Then, Danny shows up driving the bus and offers them a ride. They're about to board, when T.D. changes his mind. He sees that a nearby hopscotch game goes to eleven and there's no six, which makes him suspicious.
He and Martha then find a six in chalk on the sidewalk, outside a girl's house. T.D. asks her if she is the "Stinky" mentioned in the note, but she's just confused. He asks her if she knows about various aspects of the mystery, but her responses are all of deadpan confusion. That is, until he mentions the dinosaur, whereupon she says, "I've been expecting you".
She gives T.D. a toy dinosaur, which turns out to be his. She reveals that she tested him to make sure she had the right person, and explains how she found the dinosaur: it fell from the sky when she was on her way home from Spanish lessons, then she took it home and calculated the wind pressure to determine where it came from. She then concluded that the owner was a science-lover, which gave her the idea to make the potato sign. She then explains that she didn't put his dinosaur in the box lest it go to the wrong owner, and made the fake shopping list. T.D. introduces himself and Martha, and the girl introduces herself as Cissy, short for Cecilia. He offers to make her his sidekick, but she refuses, so he sets up a mystery for her instead (by leaving a bottle top in a box).
This episode provides examples of
- Air Quotes: T.D. puts air quotes on "grocery list" when he thinks it's coded.
- Blatant Lies: Subverted — T.D. apparently denies that Martha talks right after Cissy just heard her talk, but it turns out he was just denying that his dog spoke, since Martha is his friend's dog.
- Child Prodigy: Cissy appears younger than T.D. (so nine at the very oldest), but she was still able to calculate the wind pressure, use that to find where the dinosaur toy came from, and come up with a complicated plan to find the owner.
- Daydream Surprise: The intro turns out to all be in T.D. and Martha's imagination.
- Ditzy Genius: In the intro, T.D. is a genius, but he doesn't know how he acquired his knowledge.
- Exact Words: T.D. says it's not true what Cissy said about his dog talking... because Martha isn't his.
- Flashback: We see how T.D. found the toy dinosaur the previous day, how he lost his own toy dinosaur the previous month, and how Cissy found the toy dinosaur.
- Gender Bender: Cissy physically resembles T.D. if he were female, down to a shirt with a strawberry graphic like the tomato graphic on T.D.'s shirt.
- Gender-Blender Name: Discussed when T.D. sees Cissy and notes that she might be "Stinky", since she can't be "Rufus".
- Half-Dressed Cartoon Animal: Martha, in the intro, wears a jacket and hat but no pants.
- Imagine Spot: T.D. briefly imagines a spy leaving the message in the dinosaur.
- In-Series Nickname: Cecilia goes by "Cissy".
- Kids Love Dinosaurs: T.D. is revealed to own a toy dinosaur.
- Little Professor Dialogue: Cissy uses big words like "concluded" and "approximate".
- Mystery Episode: The episode follows the mystery of what the message in the toy dinosaur meant.
- Shout-Out: Rufus's aunt apparently lives on "Baker Street", which is Sherlock Holmes's street.
- Speak in Unison: Martha and T.D. say, "What?!" in unison when they find out the book hasn't been checked out in 50+ years.
- Stock Animal Diet: Martha wants to meet whoever would buy more hot dogs than buns.
- You Can Talk?: Upon hearing Martha speak for the first time, Cissy says, "Your dog talks!".
