Oh gee whiz boy is on his way to play baseball early Saturday morning, on his way to the sandlot he has to go pee, so he stops at McGillicuddy’s Department Store to use the john. Janitor has the men’s room closed, so Billy is sent to the ladies room because no one is in there. But when he comes out of the stall he sees in the mirror that he is now a girl. He thinks maybe going into the men’s room will reverse this, but the janitor won’t let him.
Then a mother and daughter see him/her and beckon Billy along, calling him/her 
Katie. Billy/Katie figures out she’s crossed some time-space continuum, or something. She’s living in a different city altogether, and visiting her aunt and cousin. This is a great relief to her because she is afraid of running into her friends, the ones she was going to play baseball with. They are buying clothes for Katie and her cousin, both of whom are going away to boarding school.
Katie’s cousin shares a suite with Princess Clarice of San Bugatti, but the cousin gets sick with the flu and has to go to the infirmary for a week. Katie moves in because she doesn’t like her roommates. Everyone notices how much Katie and Clarice resemble each other, except for their hairstyles. They get their hair cut the same way and then they really look like each other.
Katie and Clarice switch places when they go home for Thanksgiving vacation because Clarice wants to see what Katie’s home is like. Katie tells Clarice everything she knows about her cousin’s family, which isn’t much. Clarice was supposed to go stay with her mother visiting New York, and Katie goes in her stead. Oh boy what confusion. Afterwards Clarice doesn’t want to switch back, and pays Katie a bundle to maintain the fiction. But Katie has to be the princess in San Bugatti over Christmas vacation and go to the ball with with prince of San Marino. He tries to have his way with her, but she is only eleven. Word gets out and they have to hush it up or there will be war.
Then Katie goes back to school and Clarice isn’t there. She, as Katie, has told her aunt she doesn’t want to go back, so she doesn’t. She wants to see what a normal American school is like. So Clarice and Katie’s cousin both switch to a day school and have adventures that Katie hears about.
One day Clarice’s mother shows up and takes Katie/Clarice out of school too and brings her back to San Bugatti because there has been an assassination attempt on King Burgamond and he may not recover…
Later Katie visits America and happens to be in the same town as McGillicuddy’s Department Store. She visits the ladies room and goes to the same stall and changes back to Billy. Goes to the sandlot to look for his friends, and there they are. Where have you been? What happened to you? You throw like a girl.
