Principal Skinner introduces a fire safety skit to the students. When Ned Flanders catches on fire, the fire department tries to extinguish him. It does not work because Bart is pulling a prank with the fire hose, though water crashing out of the gym saves Flanders anyway. Skinner has Homer and Marge come to the school, where they learn Bart has been diagnosed with Attention Deficit Disorder (ADD). Principal Skinner tells Marge and Homer that Bart must take a radical, untested new behavioral medicine called Focusyn or Skinner will have to expel him.
After several failed attempts by Homer (including naming several celebrities on drugs, such as Mötley Crüe's Tommy Lee, sitcom actress Brett Butler, and actor/comedian Andy Dick, and offering Bart taffy laced with several Focusyn pills sticking out), Marge convinces Bart to take the medication. His behavior immediately improves. He begins paying attention in school and being respectful to his parents (even tutoring a Navajo boy in his spare time). However, Bart begins to suffer from drug psychosis and he starts to develop Paranoia, claiming that Major League Baseball is spying on the town using satellites. The doctors recommend that Bart go off Focusyn, but he refuses. Before anyone can stop him, he swallows several handfuls of Focusyn and runs away.
Bart wanders onto a U.S. Army base and manages to hijack a tank. He cuts a swath of destruction through the town, until he eventually stops at the school. There, he points the tank's cannon into the sky and shoots down a Major League Baseball satellite; inside are detailed documents on everybody's behaviors. Mark McGwire appears, but instead of explaining the bizarre situation, he distracts the townspeople by asking “Do you want to know the terrifying truth, or do you want to see me sock a few dingers?” (the latter choice being selected by everybody present); he accommodates them, and then hides the evidence under his cap. Marge takes Bart off Focusyn for good and puts him back on "hugs, fresh air, and good old-fashioned Ritalin". Major League Baseball, however, is still able to spy on the Simpson family due to an autographed bat McGwire gave Bart, with which he hit his 70th home run of the 1998 Major League Baseball season, which contains equipment that notes down every detail of the Simpsons' house at 742 Evergreen Terrace.