Katie Mouse's Cousin Matilda is getting married! A big wedding has been planned in Mouse Town's public park, and everyone in town is invited. Best of all, Katie will be Matilda's flower mouse, and her little brother Dylan will be the ring bearer. There's just one problem: The Games Day at Katie's school has been changed to the same day as the wedding! And Katie's the captain of her class's relay race team! Will she really have to miss it? As Katie struggles with her feelings, a mix-up with the wedding rings threatens to ruin the entire wedding. It's then that Katie discovers that only she knows how to save the day and make the wedding perfect after all.