Bart wakes up in the middle of the night and sees a UFO. He is the only one that knows about the alien invasion. Hence, he starts a journey to stop Aliens' evil plans and defend the earth. Although, this is the first Simpson themed game, the plot has little to do with the show's material. With that said, many of the show's characters make an appearance.
In each level, Bart is faced with various objectives that he has to accomplish and certain items that he has to collect before proceeding to the level boss. These objectives vary in each level, and most of them reflect Bart's personality in the show.
Bart can activate X-Ray vision to identify humans who are abducted by aliens. By jumping on those humans, he can collect letters that form the name of the level assistant. If he could form the whole name, then the level helper will provide a minor assistance in fighting the level-boss. For instance, in the first level, if Bart collects all the letters of "Maggie", she will help him fight Nelson at the end of the level by throwing Bowling balls.
There are various items that Bart can buy and use as weapons or to cause a special event. These items include Slingshots, Sprays, Rockets, Whistles, Magnets, Coins... etc. Some of the items are used in many clever ways. For instance, if you blow your whistle in front of the elder-house, Grandpa Simpson will come out and start throwing money.
The game is considered to be very difficult. Some of the jumps are nearly impossible to make from the first try, as you need to perform the jumps with precise speed and timing. The lack of saves, continues, or repeat points make it harder and more frustrating. In addition, for each life, Bart can only be hit two times before dying, and you only start with three lives and you have no continues. Finally, Most of the enemies can't be killed - you have to avoid them by making timely jumps. Don't be surprised if you are faced with the Game Over screen in the first level.
There are numerous small additions and details added to the game; for example, in the third level (The carnival), you can take part in most of the carnival activities. You can even buy a magnet and use it to cheat in the "spin the wheel" minigame.
Boss: Three humans abducted by aliens
Boss: T-Rex & Dr. Monroe
Location: Power Plant
Helper: Maggie, Marge, Lisa & Homer