Stupid Bombs are explosive weaponry built by the Koopa Troop that only appear in the Nintendo Comics System issue "Duh Stoopid Bomb!". Stupid Bombs are filled with a gas that lowers the intelligence of anyone who inhales it for a long period of time; the gas is released when a Stupid Bomb explodes, and the bombs can easily be detonated by simply dropping them or slamming them into something, including another Stupid Bomb.
Toad, acting as an agent of the F.B.I. (Fungus Bureau of Investigation), had infiltrated King Bowser Koopa's headquarters, and was listening in on a presentation in Bowser's throne room. While disguised as a Shyguy, Toad overhears Bowser, Fryguy and the red Snifit presenter discussing their latest invention, the Stupid Bomb. The Snifit claims it's ready and can make anyone "reaaaaally stupid for hours".
Shortly after the Snifit presents said Stupid Bomb, Bowser blows Toad's cover by sheer accident; while trying to escape, Toad asks the Snifit to give him the bomb, and he naively complies. Toad manages to escape from Bowser and the castle, making his way to Mushroom Land, and bursts into the castle's war room with the Stupid Bomb as Mario, Luigi, Princess Toadstool, King Toadstool, and Wooster discuss battle plans. He then trips and drops the Stupid Bomb, setting it off and making everyone present instantly dumber, except for the already-not-too-bright King Toadstool.
As the stupefied Mario Bros. are given permission by the King to mount a single-handed attack on Bowser's army, the Snifit prepares the Shyguys for combat by giving them all Stupid Bombs, saying they are perfectly safe as long as they aren't dropped. An "uh-oh!" sounds from the crowd moments later, just before a Shyguy drops one of the bombs and the explosion douses them all in its gas.
Mario and Luigi then appear, only to be mistaken for party guests and given half-a-dozen Stupid Bombs as presents. They thank the Snifit before walking away, only for some of the bombs to shortly explode. Meanwhile, the Snifit reports these events to Bowser, who seems angry at first; he then becomes happier upon realizing that Mario and Luigi will likely set off their bombs in the midst of Mushroom Land's population and render them idiotic. He then reveals the counter-agent to the Stupid Bombs, called "Smart Bombs". As Bowser explains his plan to counteract the Stupid Bomb's effects on his army, the Snifit grabs a Smart Bomb, which explodes in his hands.
The newly-genius Snifit then usurps Bowser, dumps him in Fryguy Kindergarten, and then tries to re-train the Shyguy army, who are more interested in taking the Stupid Bombs and seeing how big an explosion they can make by slamming together as many as possible. Back at the Mushroom Land castle's war room, Toadstool and company have recovered from the previous Stupid Bomb's explosion, only for Mario and Luigi to enter and bring the remaining bombs with them, which explode and render everyone (except King Toadstool) dumb yet again.