X Middle School's bathroom renovation project teeters on the brink of disaster as the new tagger "STAINLESS" strikes at will. Out of leads, Fillmore and Ingrid turn to Randall Julian, the former vandal "FLAVA SAVA", currently in solitary detention. His help puts them back on the trail, but Randall escapes, forcing Fillmore and Ingrid to hunt down both "STAINLESS" and "FLAVA SAVA" before they strike again.

The student body is in an uproar when the controversial S.A.T.T.Y. 9 tests are stolen by an assailant dressed as X's beloved mascot, Lobstee the Lobster. Fillmore and Ingrid are under pressure—they have only 48 hours to find the tests or Principal Folsom will have the entire school take the tests again.

The library's books have disappeared, and Tony Clementina, X Middle School's local heavy, is all too eager to take credit for it. Meanwhile, an empty can of Carbee Cola leads Ingrid back to the biggest custard spill in X Middle School history, an incident that disturbingly points to Fillmore.

During an undercover sting operation, boxes of band candy were accidentally discovered in the lake. The candy is traced back to the Red Robins, seemingly a school club in the same manner as Girl Scouts who make record sales of their award-winning taffy. In reality, the Robins strong-arm their competition, and the Safety Patrol have been trying to catch them in the act for a long time. Ingrid goes undercover as a Red Robin, but Vallejo isn't convinced she's the right person for the job and suspects something might go wrong.

After accidentally destroying Folsom's birthday party, Fillmore and Ingrid are asked to do convention duty. However, the train convention experiences sabotage, leaving the entire convention ruined. Fillmore and Ingrid first suspect the RC Car Club to be behind it, but the evidence points in the other direction.

Duappy, a virtual pet kept alive the longest, has been stolen, and it is up to Fillmore and Ingrid to find X's pride and joy before a reporter comes to X to do a story on Duappy and before Duappy dies.

Fillmore's partner moves away to Tennessee, and he is partnered up with Brad Parnassus, the smartest kid at X Middle School. Rebellious and super-smart new student Ingrid Third doesn't take her introduction to X student body well at all, and a giant stink bomb soon explodes in the hallways.

Mrs. Cornwall's pre-algebra final exam looks like it's going to be a tough one. However, some of her students have decided to take more extreme measures beyond studying. Fillmore sleeps over in the school to safeguard the exam, but an unexpected blizzard leaves him standing alone to face a desperate gang of determined exam thieves.

Mrs. Lawson's field trip to the Modern Contemporary Natural History, Art, Science and Miniature Museum takes a horrifying turn as someone draws a mustache on the priceless painting "The Lobster Man at Port". Fillmore, Ingrid, and O'Farrell work to find the culprit and save the painting, but the clock is ticking and everyone's a suspect.

Scooter thefts are up, and the Safety Patrol faces a challenge from student activist Derek Minna, who establishes B.A.G.A.S.T. (Boys And Girls Against Scooter Theft), a scooter and rollerblade equipped task force that he intends to ultimately replace the safety patrol. Fillmore and Ingrid trace the thefts to the Metal Shop and infiltrate the gang in an attempt to take down the culprits.

X Middle School's best hope to break a losing streak is Checkmatey, a flamboyant rapper who is X's finest chess player. However, Checkmatey is kidnapped before the championship match, and multiple suspects possess a suitable motive for the crime. Fillmore and Ingrid are on the case—but the clock is ticking down and Checkmatey is nowhere to be found.

The heat is on from Student Council Representative Peabody to get illegal term papers off the street, but the perps always seem one step ahead of the Safety Patrol. After a botched raid and the sabotage of his bike, Fillmore turns to Sonny Lombard, his old partner from his bad-boy days. They track the papers back to Rudy Teravall, the Kingpin of Home Economics, but just as Fillmore and Ingrid get ready to move in on Rudy's gang, Peabody gets tipped off about bootleg hall passes in Fillmore's desk, resulting in Fillmore getting suspended from the force. Nonetheless, Fillmore is determined to find the term papers—with or without his badge.