The Tower of Dr. Zalost

A giant moving tower on wheels and (computer generated) legs shoots magical cannonballs that turn everyone in Nowhere green and sad. It is the work of the green-skinned Dr. Zhalost who wants to make everyone in Nowhere miserable like he is. He demands 33 1/3 billion dollars from the mayor. His assistant is a hunchbackish rat with a scar called Rat. Zalost gets the money, but he is still not satsified.
Meanwhile, Muriel is making happy plums. Eustace is mad the paper isn't delivered so he tries to get it, and after a failed attempt, he meets up with Zhalost's tower. It follows Eustace and fires cannonballs. Eustace isn't effected because he doesn't have much feeling. But Muriel is hit, so Courage needs to find a way to make her normal.
Courage hides in a pizza box that Eustace delivers to Rat. Courage finds piles and piles of cannonballs and he accidently realeases some and one hits Rat. Rat turns into a giant demon rat. Courage fires happy plums because Muriel said...

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S03E01 - Muriel Meets Her Match