{Source : Wiki}

Introduced as the Emperor's advisor, Yzma is first seen in the movie sitting in Emperor Kuzco's throne and making decisions for him behind his back. Kuzco discovers that she has been meddling in his affairs, sees her doing it again, and without thinking, he fires Yzma as a result.
Later on, she vents her rage by smashing busts of Kuzco (whom she has now turned on and who she claims to have raised thus being his adoptive mother) with a giant hammer (with Kronk, her right-hand man, putting out more and more busts for her to destroy). However, a careless comment by Kronk gives her the idea to kill Kuzco with Kuzco out of the way and no heir to the throne, as well as her then-recent firing not being fully disclosed to the public yet, Yzma would automatically take over the position and rule the empire. She and Kronk go to Yzma's laboratory known as the Secret Lab using a roller coaster, where Yzma decides to turn Kuzco into a flea, then put that flea in a box, put that box inside of another box, mail that box to herself, and upon its arrival, smash it with a hammer; a plan she announced to be brilliant (just before accidentally knocking the flea concoction into a nearby flower, destroying it instantly), but in order to save on postage, she decides to just poison Kuzco instead. She arranges to have a dinner with Kuzco that night.

That evening, Yzma and Kronk get everything ready for dinner (including the potion used to kill Kuzco). Suddenly, Kuzco slams the door open, and Yzma tells Kronk to get the Emperor a drink. Even though Kronk's more interested in cooking dinner than carrying out Yzma's orders, he tells her that the poison's ready. Kronk poisons Kuzco's drink, but he forgets whose drink is whose, so he mixes all three drinks together. And he and Yzma stealthily throw away their drinks as Kuzco downs his. Even though it may look like the Emperor was killed at first, he suddenly perked back up and started turning into a llama. So Yzma orders Kronk to knock out Kuzco with the bowlful of broccolis, take him out of town, and finish the job. However, when Kronk asks for dessert and coffee, Yzma replies with, "All right. A quick cup of coffee. THEN TAKE HIM OUT OF TOWN AND FINISH THE JOB!!!"

Taking the llama's unconscious body to the edge of the city in a big red bag, Kronk throws Kuzco into the river, but after talking to his conscience, Kronk has a change of heart at the last minute and rushes to save Emperor Kuzco before he falls to his apparent death, and succeeds. Walking down the stairs and asking himself what he should do with the body, Kronk accidentally steps on a cat's tail (thus waking it up) and lets go of the bag, which falls down the stairs and onto the cart of a fat peasant named Pacha, just as he's leaving the city with his pet llama Misty.