S19E05 Treehouse of Horror XVIII
In "ET: Go Home," Bart finds Kodos in the back yard and they become friends. However, Kodos has plans of killing everyone. In "Mr. and Mrs. Simpson," Homer and Marge discover that they are both expert assassins and try to kill each other. In "Heck House," Bart, Milhouse, Nelson, and Lisa pull pranks on Springfield residents while trick-or-treating, and are subjected to the wrath of Ned Flanders and his 'Heck House.'