The leaf collection was a project that Calvin had to do for school. He had to collect fifty leaves total, and Miss Wormwood said that every leaf had to be a different kind (meaning that he couldn't have two or more of the same leaf). Calvin believed it was impossible to get it done in two weeks, because he wasn't willing to work weekends or spend beautiful days doing work for school. Susie was almost done with hers, since she pretended to make it a game. (This was hardly Calvin's idea of fun, though.) Calvin did have an idea to visit the Arboretum with his mom, but the idea apparently didn't come to him until the last second, while Calvin's mom started fixing dinner. Calvin and Hobbes are next seen gathering leaves, with Calvin complaining that his mom wouldn't take him to the arboretum, assuming that she is the reason he gets bad grades. Hobbes reminds him that he waited until the last second, Calvin claims that that was when he thought of it, and blames his mom's apparent inflexibility as the "reason" for his bad grades. Frustrated as ever, Calvin wished for a way out of the assignment. At that moment, a UFO, piloted by Galaxoid and Nebular, landed close to where he and Hobbes were standing. The aliens told Calvin they were taking the Earth over. Seeing an opportunity, he decided to give the world for fifty different alien tree leaves - properly labeled and identified. Galaxoid and Nebular got to work, but arrived late at night, which resulted in Calvin getting discovered by his dad. The next day, Calvin presented his collection of alien tree leaves. Unfortunately, no one, not even Miss Wormwood believed the leaves were from out of this world, because they all resembled maple leaves cut into odd shapes.
Miss Wormwood declared that Calvin had made a total mockery of the assignment, and so failed him. Upset, Calvin took a stroll through the woods, complaining and considering leaves "useless knowledge". Hobbes pointed out that the leaves Calvin had just pulled away from a tree were Poison Sumac.