Anais is playing puzzles with her mother. She is bored of them since they are so easy for her. She sees Gumball, Darwin, and Richard playing a game using their butts, and she decides that she wants to join in on their dumb antics. When she asks Gumball and Darwin if she can learn, they agree to teach her how to be "dumb."
The first game the three play is called "I Am The President," which involves a president ordering everyone else to do various tasks, like getting a very stinky sock, using it as a flag, and kissing it (and choking). Anais does not understand and they stop playing. Nicole then comes to the window and offers Anais a game of puzzles, which prompts her to try and keep learning.

Then they get ready for the next activity. Gumball asks Anais if she has her notebook ready. Gumball then gets her to write Rule #1: "Dumb people do not take notes." Anais writes what Gumball said, but as she finishes, Darwin snatches her notebook and pencil, writes something in it, hands it to Gumball, who throws it out the window, onto the windscreen of the Robinson´s car, blinding Mr. Robinson, and making him crash into a fire hydrant.

The next activity is learning to speak dumb, which, as a pretend language, consists of the word "dumb" said in multiple ways. Gumball and Darwin demonstrate. Anais tries, but ends up saying "something very rude" and does not understand. Then Gumball and Darwin sang a song that consists of the word ´dumb´ It seems to have some resemblance to the Gumball theme at the start. She is frustrated, but she decides to try yet again.

Gumball and Darwin decide for Anais to try and make a sandwich the "dumb way". She starts about by slathering the mayo on the bread, but Gumball and Darwin laugh and state that it is wrong and demonstrate yet again. Anais is even more frustrated and does not know why dumb people act the way they do.

The three go outside for the next activity, but Anais states that she cannot understand the way dumb people think. She realizes that they don't think, and has an epiphany. Richard comes outside, offering a game of Fruit Ball, which Anais wishes to join. Richard is surprised, but lets her play. The four have dumb fun. Then they go to bed.

The next day, Gumball wakes up, only to find himself as the only one still in the bedroom. He finds Darwin and Anais toe-wrestling with Richard in the backyard. There is no room for him, as Richard only has two feet, there is already a judge, spectators, and anything else. Gumball goes inside, being repeatedly left out of dumb activities, and being driven to the point of playing puzzles with Nicole. He cannot solve the puzzle, even with only one piece left to use. Gumball and Nicole awkwardly part ways, and Gumball decides to get his dad back from Anais.

A "dumb race" with blindfolds is organized, which involves the person who ran furthest away from the start being the dumbest, and opening your eyes is cheating. Idaho, Banana Joe, Rocky, Anton, and Richard are out, Richard being accidentally proposed to by Larry, who was having a picnic with his girlfriend. Anais, Gumball, and Darwin are still in, Gumball upset at Anais the entire way. Anais says that she only wanted to be dumb for a day, which makes Gumball feel better. They make up, stopping just before a highway, and get run over by Bert.

In the hospital, Rocky, Banana Joe, Idaho, and Anton are in beds, as well as Gumball and Anais. Dr. Butt comes to see Gumball, who looks remarkably like Richard's butt (despite not being it), and Nicole and Richard see the children. Gumball is reinstated as the dumbest person ever, and Anais is out as she does not want to be dumb anymore. Richard states that the dumb race happened all because of kids competing for his affection, when his favorite is Darwin. Nicole gets mad, and Richard adds on "favorite fish". The family then notices Darwin is missing, when he is still running the dumb race, asking if he's winning, then running into a tree.