After Madame Foster suddenly dies, her granddaughter Frances "Frankie" Foster is left in charge of Foster's Home For Imaginary Friends. At the same time, Mr. Herriman retires from being president of the home due to the wants of his deceased creator. Frankie is used to running Foster's, but being the sole owner is on a completely different level.
Set eight years after the cartoon, Frankie Rules is a Foster's Home for Imaginary Friends fanfic that tells of what could happen to the cast after Madame Foster's death. It's mostly humorous with some light romance and drama elements sprinkled about.
Frankie Rules provides examples of:
- Accidental Misnaming: Even after eight years, Bloo still thinks Berry is named "Heather".
- Adaptation Name Change: Goo's real name is "Gunilla".
- Adaptation Relationship Overhaul: In the show canon, Frankie forgives Goofball when he shows her genuine gratitude for taking care of him at the end of the episode. In this story she instead has hatred for him.
- Crossover: One chapter reveals Mac is related to Tommy from Robotboy, who has also been aged up like Mac. It also reveals that Robotboy eventually did become a real boy (fittingly named "Robbie"), and Bloo thinks he's a robot who "has batteries up his ass" (he's right for once), and that Tommy and Lola are an Official Couple.
- Dark and Troubled Past: It's implied that Berry has one, but it's otherwise not touched upon.
- Death by Adaptation: Madame Foster, obviously.
- Do Not Call Me "Paul":
- Goo's first name is "Gunilla" but she hates being called that.
- Frankie isn't fond of being called "Francis".
- Honor Before Reason: Despite any past crimes, any imaginary friend that come to Foster's is welcome.
- I Can't Believe It's Not Heroin!: Mac intentionally gets himself sugar high for kicks.
- Last-Name Basis: Herriman changes from calling Frankie "Miss Francis" to "Madame Foster" after she becomes the new owner of Foster's.
- Laser-Guided Karma:
- Bendy and Goofball finally get some comeuppance for their antics, with Wilt and Frankie both coming down on Bendy when he tries to pull his Wounded Gazelle Gambit shtick and Goofball getting assigned as Count Flatula's roommate.
- After Mac and Goo secretly have sex at Foster's multiple times, leading to the Friends asking very awkward questions, Frankie lets the friends tear the house apart and then assigns her employees to clean it all up by themselves.
- Must Have Caffeine: Not outright stated but implied for Mac given that Frankie and Goo both talk about Mac's morning coffee as something normal and harmless. Bit weird given the way he reacts to sugar, though it's possible he reacts to caffeine differently.
- Mistaken for Romance: Bloo, being Bloo, believes that Wilt and Frankie are having a Secret Relationship. It's a bunk rumor.
- Named by the Adaptation: Madame Foster's given name is "Martha". It's worth noting that this was what people thought her first name was at some time, because in "Neighbor Pains", it sounded like Old Man Rivers addressed her as Martha. This was debunked when he was actually saying "Foster", and you can even see Rivers' lips forming an F when he says this.
- Original Character: Eric Johnstone is an imaginary friend-focused psychiatrist and a love interest to Frankie.
- Plot-Triggering Death: Madame Foster's death means Frankie's in charge of Foster's.
- Related in the Adaptation: Tommy from Robotboy is Mac's cousin.
- Secret Relationship: Eurotish and Eduardo are secretly together.
- Shipper on Deck: Madame Foster went through the trouble of setting up a situation to get Mac and Goo to confess to each other.
- Strong Family Resemblance: Goo is instantly able to tell that Mac is related to Tommy because their heads are similarly shaped.
- Time Skip: The fic takes six years in the future. It also features frequent small timeskips between scenes and chapters.
- What Measure Is a Non-Human?: A minor plot point as things like therapy for imaginary friends are only recent developments.