"The Incredible Shrinking Gadget" is the 8th episode of the second season of Inspector Gadget.
Everything seems relatively normal at the convention and Gadget doesn't even notice that he's already been on the receiving end of the first of 4 treatments that will shrink him to the size of a mouse. Conventioneer (and M.A.D. agent) Professor Doomkauf is the culprit, and wielding his miniaturizing ray gun, he follows Gadget home to finish the job. Aware only that his clothes seem a tad larger than normal, an unconcerned Gadget takes a nap on his bed.
When he wakes up another 2 sizes smaller, he realizes that something's amiss ― but attributes it to being held prisoner in an over-sized house. Doomkauf hasn't fared too well himself ― he can't seem to keep the ray gun in his possession and has been reduced to the same tiny size as the inspector. However, small or not, he still intends to shrink Gadget even further. While watching over his master has always been Brain's forte, he's really got his work cut out for him this time: The impatient Capeman sees the dog as an enormous nuisance and keeps thwarting Brain's protective efforts by kicking him out of the house. Meanwhile, Penny has sneaked into Doomkauf's lab to get the shrink ray's antidote. It is locked in a safe, of course, but she manages to bring it home and restore her uncle to normal size just as Chief Quimby appears. So for now, Doomkauf escapes in Penny's toy plane, but he'll be back to tangle with Gadget again.