The Third Dimension was a strange world behind the bookshelf in the Simpsons' house. The only living creatures there seem to be three dimensional goldfish, who swim in a three dimensional lake. There are mysterious three dimensional shapes here, such as cubes, cones, cylinders and spheres. There appears to be various maths sums floating in mid air. There is also a museum in the third dimension, as well as a large sphere resembling Jupiter.
Homer enters the Third Dimension while searching for a place to hide from Patty and Selma, and looks behind a bookshelf. He discovers a portal here, and runs into the world of three dimensional objects. Homer creates a huge black hole, which devours the shapes, the fish, and the museum. Bart runs into the Third Dimension to help Homer, only to discover that the black hole is now huge. Bart climbs onto a signpost and tries to reach Homer. Homer attempts to jump over the black hole, but fails, and is warped to the real world. However, Bart is saved before the dimension implodes.