Mr. Cake notices something odd one morning: they had lots of diapers last night, but now they have barely any. Mrs. Cake suggests that Diamond Tiara took them when she turned into a toddler, but Mr. Cake says that that can't be it as she only took the one diaper, so the diapers must be stolen.
Mrs. Cake suggests that Pound is the thief because he still is a bit reluctant to use his potty, but Mr. Cake points out that if it was Pound, he'd be stealing the whole box rather than stealing them one by one. Mrs. Cake half-jokingly suggests it was Pinkie Pie because she wonders if the shock from the horn flu incident is making her incontinent or something, but Mr. Cake points out that Pound's diapers wouldn't fit her and he didn't notice anything unusual in her room (which he likely would if she were incontinent).
The parents ask the twins if they know who did it, but they don't, so they decide to have Diamond Tiara, who will be foalsitting the twins that night, look for the thief. Mr. Cake points out that the diapers could fit both colts and fillies, and that they could stretch to fit an older foal, and Mrs. Cake wonders if Diamond Tiara is the thief due to missing being a toddler.
That night, Diamond is asked to catch the diaper thief if she can, but not to let that get in the way of making sure the twins behave and use their potties. A "mysterious figure" spies on Diamond and the twins, and when they go for a potty break, comes in, but makes noise and Diamond comes down.
The thief then reveals herself to be Apple Bloom. Diamond asks her why she stole them, and Apple Bloom, embarrassed, reveals that seeing as her parents died before she was potty trained, she thought wearing a diaper would help her remember them. The fillies agree to keep it secret.
This chapter provides examples of:
- Adaptational Explanation: Bright Mac and Pear Butter's absence is explained by them being dead.
- Babysitting Episode: The twins get foalsat by Diamond Tiara again.
- Everyone Has Standards: Even when she was a bully, Diamond Tiara never stole from a baby.
- Goo Goo Getup: Apple Bloom tries to remember her toddlerhood by wearing diapers.
- Mistaken for Thief: The parents briefly wonder if it was Pound, Pinkie Pie, or Diamond Tiara who stole the diapers, when really it was Apple Bloom.
- Mystery Episode: This chapter is about the Cakes trying to find out who stole the diapers.
- Potty Failure: Discussed when Mrs. Cake wonders if it's Pinkie who's taking the diapers because the shock from Starlight's spell may have had "effects" on her.
- Power Incontinence: Pumpkin points out that her magic only works "in short bursts".
- Pun: Diamond Tiara tells the twins, "No buts, you know besides the ones you're gonna sit on your potties."
- Tickle Torture: Discussed when Diamond threatens to tickle the diaper thief if she doesn't give back the diapers she stole.
