When X Middle School takes delivery of a brand new set of bathroom fittings, a graffiti artist going by the moniker Stainlesss strikes. Fillmore and Third need help, and they get it in the form of Randall Julian aka Flava Sava, a gifted former vandal now in a 'permanent state of detention'.
Completed copies of a controversial aptitude test are stolen from X Middle School. Fillmore is on the case, but his partner Third is conflicted by the crime - she doesn't think it was such a bad thing that the tests went missing.
When the X Middle School library is robbed of its most popular titles, Fillmore is convinced that petty thug Tony Clementina is guilty - he's a lost cause. Meanwhile Third investigates a cold case - the world's largest ever custard spill - which reminds Fillmore that no cause is ever truly lost.
X Middle School's mascot, Lobstee the Lobster, is stolen. Fillmore and Third are having no luck, so Principal Folsom brings in the school's resident psychic, Allastair Greystone. But Allastair seems to know a little too much about the theft.
The Red Robins are a ruthless extra-curricular club who strong-arm other school societies out of the confectionery retail business. Third goes undercover, but Jr Commissioner Vallejo is worried that she might be enjoying the roe a bit too much.
The world's longest surviving virtual pet - Duappy - belongs to X Middle School pupil Everitt Konquist. But days before Everett was due to appear on a television show, Duappy is kidnapped. Everett is heartbroken, but his next-door neighborly Lorraine is eager to help him through it.
Fillmore writes a letter to his former partner, now living in Tennessee, about his current partner, Ingrid Third, and how she was almost instantly expelled from X Middle School, before joining the Safety Patrol.
Fillmore believes that a group of pupils are planning to steal an algebra test during a Snow Day, but the algebra teacher Miss Cornwall prefers to think better of her class. Fillmore keeps lookout at the school anyway, but he is severely outnumbered.
During a field trip to a local art gallery, a priceless painting is vandalized. Fillmore and Third have a limited window to find the culprit - and the pen responsible - before the damage becomes irreversible.
X Middle School is plagued by a spate of scooter thefts, and ambitious student council member Derek Minna sets up a vigilante force to stop them. Meanwhile Fillmore and Third go undercover with the scooter thieves to try and uncover the mastermind behind the thefts.
Illegal term papers are being distributed, and Fillmore comes under pressure to solve the case. Student Council Representative Peabody doesn't trust him, especially when Fillmore recruits the help of his old partner in crime Sonny Lombard.