The tree house collapses under the weight of snow on the roof. After meeting Frank Gehry at the Sugar Bowl, Arthur and his friends come up with different ideas to build a new one. The only design they agree on is the original, which they rebuild. Frank Gehry guest stars as himself.
The school choir is excited about singing in Crown City, but they aren't excited about Ms. Krasny's replacement, Dr. Frederique Fugue. All goes well until the road is closed and the trip is canceled. They stop at a diner and sing anyway.
After being unable to fit into a suit for his play (and instead, is given a "husky" suit for the role), Arthur looks at a variety of diets. He finds that exercise is the best way of keeping fit, as opposed to diets.
Arthur aces five quizzes in a row. But his luck runs out when his mom takes a new job. At first, he and D.W. get all worked up over this change at home, but they learn that they'll always be priority #1.
The community pool is closed for repairs, but Arthur and D.W. can use the MacDonalds' pool. Arthur discovers that James' older sister is Molly. Eventually, they get along but avoid each other in public because their friends won't approve.
Grandma Thora finds a stray dog named Killer. She is a ferocious dog that barks at and bites other people and pets. Grandma Thora finds out that she has been in and out of the pound several times and changes her mind after leaving her there. She rethinks her behavior toward others.
The Brain and Francine feel that their soccer team is doing stuff that is much too easy for them, so they join the Elwood City All-Stars. They practice hard, until they have a soccer game against their old team, but they become extra players for it.
D.W. finds a ball, drops it, and finds Emily holding it. Emily tries to make up an excuse to keep it instead of D.W. She lies but it keeps getting bigger as she tells more lies, such as when she said Marie-Hélène had a terrible disease called halitosis. She has a dream and tells Marie-Helene and D.W.
The Brain is convinced that his parents' Breezy Listening Blues CD is the reason why he didn't do as well as usual on a test. He also has to find some music that will suit everyone to play in their ice cream shop.