Ellie Blake welcomes us to her unbelievable yet one-hundred-percent true story ("Prologue"). It's a crazy day at the Blake household: Ellie's mom, Katherine, is busy preparing to cater her own wedding, which she hopes will land her a cover story in Down the Aisle magazine. Her sous chef, Torrey, and a staff of Cater Waiters are frenzied trying to complete every last task. Meanwhile, Ellie desperately wants to join her friends, Karl and Monica, on the Hunt - an annual, epic scavenger hunt at their high school that is sure to elevate their status amongst their classmates. There's just one problem - tonight is the rehearsal dinner ("Just One Day").

When Fletcher, Ellie's magic-obsessed little brother, runs through the house brandishing an antique hourglass that Ellie's father gave her before he passed away, a fight ensues amid the chaos. Frustrated by her mother's lack of sympathy and unwilling to let Mike, Katherine's fiancé, play a fatherly role, Ellie lashes out at the whole family. When Katherine privately confronts Ellie about her attitude, both are firm in their resolve, neither truly understanding or listening to the other. The hourglass begins to glow as the two women proclaim their wish that the other would "understand and see the world my way for just one day!" ("The Hourglass") In a flash, Ellie and Katherine both find themselves inside the other's body.

Katherine and Ellie panic and drop the hourglass, which smashes to pieces on the floor. Ellie (now in Katherine's body) recalls that there were two hourglasses - a matching set given to her and Katherine from her father. All they need to do is go get the other one and they can switch back. Unfortunately, Katherine admits that she sold the other hourglass to an antiques store to help fund her catering business and keep the family on its feet. The two realize they must spend the day as each other until they can buy back Katherine's hourglass ("I Got This"). Katherine heads to school and encounters various Students, including Adam, the Listmaster for the Hunt, and Savannah and Kitty, the intimidating apex of the high school social hierarchy and her sidekick. Katherine begins to realize that she has no idea what she's in for.

Back at the Blake house, Ellie tries to fake her way through the interview with Danielle and Louis from Down the Aisle magazine, which doesn't go so well when the wedding cake ends up all over the floor ("Wedding Tango"). At school, Katherine finds her way to biology class, where Adam is her lab partner as they dissect frogs. Confident in her dissection skills as an adult, but confused by her teenage hormones that Adam ignites, Katherine struggles to keep her focus ("Oh, Biology").

At a mandatory parent-teacher conference, Dr. Ehrin, the principal, introduces Ellie's teachers, Señor O'Brien and Mrs. Luckenbill, who note that Ellie has an attendance problem dating back to around the time her father passed away. Katherine-as-Ellie ends the conference with a new compassion for her daughter's situation, but furious at her daughter's decision to skip class, leaving the teachers bewildered.

Ellie and Fletcher head to the antiques store, only to find it has gone out of business, the inventory spread around dozens of other antiques stores in the city. Fletcher suggests they can hunt for it at another store, but this gives Ellie an idea - get the hourglass on the list for the Hunt and the whole school will be searching for it ("I Got This - Reprise"). Ellie calls Katherine to give her the update, and in gym class ("Watch Your Back"), Katherine attempts to team up with Savannah (who is clearly going to win the Hunt) and tries get Adam to add the item to the list. Being a paragon of integrity, he refuses, since she would know the clue ahead of time, and Savannah laughs in her face. Karl and Monica witness her attempting to sell them out and storm off, hurt and upset.

In the car on the way home, Fletcher confides that he is excited to move to Hollywood to be a world-class magician, a career Katherine encouraged him to explore. Forgetting herself, Ellie explains how "Parents Lie" when they tell you you're special and that they'll always be there - they won't. Upset by this revelation from his "mother," Fletcher runs into the house, where Guests are starting to assemble for the rehearsal dinner. When Ellie finds out that her best friends are mad at her because of Katherine, she lashes out at Torrey and tells Mike she's reconsidering getting married. But before Mike can leave, he finds a note left by Fletcher, who has run away from home. The entire rehearsal dinner party searches for Fletcher, while Ellie panics in Katherine's body, and no one listens to Katherine, who is trying to get the search organized from Ellie's body ("I'm Not Myself Today").

While posting clues for the Hunt, Adam encounters Fletcher alone at a bus stop, and, convincing him that his family might not be that bad, offers to take him home. The two arrive back at the Blake house just as Officer Kowalski questions Katherine and Ellie about what might have caused Fletcher to run away. Ellie-as-Katherine smooths things over with Adam and convinces him to add the hourglass to the Hunt. She also repairs her damaged friendship with Karl and Monica, and they agree to let Ellie re-join them in the Hunt. Ellie apologizes to Mike and Fletcher for the wreck of a day. Each forgives her, and Ellie begins to feel like she's not a failure after all…

As the Hunt commences ("Go"), kids race all around the city collecting items, performing stunts, and taking pictures. Karl and Monica are terrified by a group challenge - a dance-off - but Katherine convinces the kids to stop caring what they look like and enjoy being the perfect teenagers they are. Realizing that it's a joy to live in-the-moment, Katherine and the team rush off to find the hourglass.

Early the next morning, Katherine, Monica, and Karl stumble upon an antique watch shop and see the hourglass in the window. They wake the owner, Mrs. Time, and convince her to let them borrow the hourglass. Just then, Savannah appears and a tug-of-war ensues. Katherine realizes that she can't control every situation with carefully planning and strategy. Finally letting herself go, she stomps on Savannah's foot and wrenches the hourglass from her grip, running to the finish line and winning the Hunt ("Tug of War").

Later that morning, Katherine returns home to find Ellie in her wedding dress, ready to walk down the aisle. The two hold on to the hourglass ("The Other Hourglass") but can't get the magic to reverse. Terrified, Ellie and Katherine fear that they will be stuck in each other's body forever. Seeing no other option, Katherine tells Ellie to walk down the aisle and break up with Mike. Ellie stops the Minister and tries to do so, emotionally confessing that she felt broken for years after her father's death and resented her mother for it. But now she realizes that her mother only wanted to protect her because she loves her. Magically, the hourglass begins to glow, and as the two articulate their newfound understanding and empathy for each other, the magic triggers and mother and daughter find themselves in their proper bodies ("Today and Ev'ry Day"). With a deeper understanding of herself and her mother, Ellie celebrates the lessons she's learned with her family and friends ("At Last It's Me").