Brother's Little Helper is the second episode of Season 11 (originally going to be in season 10 in production order).
After Bart continually wreaks havoc in school, Principal Skinner recommends to Marge and Homer that Bart take some behavior medication. The drug initially shows promise, but it quickly alters Bart's psychosis and turns him into a paranoid conspiracy theorist.
During Fire Safety Day, the Springfield Volunteer Fire Department arrives at the school to teach kids about fire safety. Although Lisa has a genuine interest in fire safety, Bart namely asks for the names of ire safety equipment namely to have Principal Skinner utter double entendres. Principal Skinner introduces a fire safety skit to the students. The skit consists of Ned and Maude playing a hippie couple who use drugs. During the skit, Ned catches on fire as part of the act, but it goes awry when his life is endangered due to it spreading to the extent that not even the stop-drop-and-roll method worked ("It's not working, it just spreads the flames!"). The fire department tries to extinguish the flames, but they can't do it because Bart used the fire hose to flood the gym. Skinner has Homer and Marge come to the school. It is then when Bart is diagnosed with Attention Deficit Disorder. Skinner recommends that Bart take a behavior drug called Focusyn, and states that the only alternative is that he will expel Bart if his parents don't give him Focusyn.
After several failed attempts by Homer, Marge convinces Bart to take the medication, and his behavior immediately improves. He begins paying attention in school and being respectful to his parents. However, side effects soon occur as Bart becomes paranoid that Major League Baseball is spying on the town using satellites. The doctors recommend that Bart go off Focusyn, but he refuses. Before he can be stopped, he swallows several handfuls of Focusyn and runs off.
Bart wanders onto an army base and manages to steal a tank (a la Shawn Nelson). 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, proving his theory right. Mark McGwire appears, but instead of explaining the bizarre situation, he distracts the townspeople by hitting some baseballs and then takes off with the evidence. Marge takes Bart off Focusyn for good and puts him back on "good old Ritalin", which he is okay with.