In Total Rickall, what kind of memories can the alien parasites create?

In “Total Rickall,” the alien parasites can create positive memories. That rule lets Morty distinguish the real members of the Smith household from parasites, because his genuine memories with Rick include unpleasant experiences that the parasites cannot implant.

The season 2 episode begins when Rick shoots “Uncle Steve,” revealing that the apparently familiar relative is an alien parasite. The creatures multiply by inserting elaborate false memories and then appearing as friends, relatives and acquaintances from those memories. Soon the house is crowded with characters such as Cousin Nicky, Sleepy Gary, Mr. Beauregard and Tinkles the fairy lamb.

The positive-memory limitation is the episode’s central deduction, but it is not foolproof emotionally. Beth has no negative memories of Mr. Poopybutthole and mistakenly shoots him, even though he is real. “Total Rickall” aired on August 16, 2015, was written by Mike McMahan and directed by Juan Meza-León. It is often called a clip show because of its many flashbacks, though those apparent memories are deliberately fabricated parts of the plot.