At a science fair, Bart wanders into a virtual reality exhibit and must pass all the "virtual worlds" (levels) to escape the machine. Passing a set of virtual worlds grants an ending sequence, and completing all the levels without losing all lives remaining wins the game. The format is similar to the previously released Bart's Nightmare, save for the lack of a hub world from which the levels were accessed. Gameplay for most of the levels consisted of standard platforming elements; others took advantage of the SNES' Mode 7 abilities. The levels were as follows:
Dinosaur: Bart is a dinosaur that somewhat resembles a coelophysis, on top of Mt. Springfield in the primeval era. The player must survive the various platforming hazards while fighting off other dinosaurs and neanderthalic versions of various Simpsons characters. Homer, as a caveman, is particularly persistent, and appears as a boss along with Moe, attempting to knock Bart into a freezing lake. Sending the two into the lake ends the stage; an ending is played of a frozen Homer on display at the Springfield Museum of Natural History, sighing "D'oh" as Dr. J. Loren Pryor passes by with a plateful of donuts.
Baby: Bart "relives" his infancy, escaping from his bedroom at the Simpsons' residence to explore. The player must guide Baby Bart through complex platforming sequences while avoiding hazards and animals. Bart eventually ends up in a live circus act, and is safely launched back home via cannon without his parents realizing his absence (at least Marge).
Pig: Bart is an escaped pig at Krusty the Klown's pork factory. The player must rescue Bart's swine brethren while again surviving platforming hazards and fending off clowns and corporate thugs. After defeating the owners of the factory, an ending sequence plays of Pig-Bart freeing all the captive pigs.
Water Slide: Bart slides down a massive, surreal waterslide tube at a waterpark, dodging hazards and other waterpark patrons. The slide sometimes forks, with only one path the correct one. Taking the wrong fork will either force the player back (courtesy of a pudgy Homer blocking the way), or ejected from the slide into a bizarre death such as falling off a cliff or being eaten by a lion. Reaching the end of the level grants an ending sequence of a triumphant Bart soaking the twins Sherri and Terri. Blinky the Three-Eyed Fish makes a cameo as both a hazard and a marker for the level's progression meter.
Vandal: Bart, from his hiding spot, must hurl tomatoes and eggs at his Springfield Elementary classmates, striking each one. Running out of time and/or throwing objects with classmates remaining results in failure, as does hitting any passing adult (the exception is Principal Skinner, who can be hit in the posterior when he is bent over).
Post-Apocalypse: A spoof of the Mad Max films, Bart is a road warrior attempting to survive a post-nuclear holocaust Springfield. The level plays similarly to such game classics as Road Rash and Mach Rider. The player bikes through a highway while fighting off persistent attacks from Jimbo Jones, who throws bombs at Bart from his ATV, and the more troublesome Kearney, who attempts to beat Bart with a chain when he pulls alongside him, as well as ramming Bart's motorcycle with a chariot-like vehicle. Bart may defend himself by kicking the bullies when along side their vehicles, or by shotting water balloons when behind them. Otto and his bus also act as an obstacle, driving the opposite way of Bart in an attempt to hit the player, which causes tremendous damage. Completing the level sees Bart return to his home on Evergreen Terrance. Bart's family is revealed to have been long dead, reduced to a group of skeletons on the living room couch. An uncaring Bart shoves their bones out of the way as Krusty's post-apocalyptic TV show is about to air ("Hey, surviving kids!").
If the player runs out of lives, game over occurs (with Bart getting embarrassing nausea from the virtual-reality machine). If the player successfully passes all of the levels, a final ending sequence shows Bart safely escaping the machine, while a gullible Homer tries the machine himself to immediate disaster
Source: Wikipedia, "Virtual Bart", available under the CC-BY-SA License.