When summer arrives, Judy Moody is buzzing with excitement, dreaming up all kinds of wild adventures, until her plans take an unexpected turn: her parents announce a trip to California, leaving her stuck at home with Frank Pearl, her second-best buddy. Determined to make this summer one for the books, Judy hatches a brilliant plan—a "Thrill Points" competition. Every daring activity they check off her list earns them points and a chance to usher in the most exhilarating summer ever.

As luck would have it, Judy's eccentric and spirited Aunt Opal rolls into town just as her best friend Emily jets off to Borneo, and the notorious Mrs. Grossman is detained due to her continuous skirmish with the authorities. With Aunt Opal's infectious zest for life, Judy finds a partner-in-crime ready to embark on a series of audacious escapades. Together with her mischievous little brother Stink and an eclectic crew of friends, they set off on a whirlwind of outrageous summer missions.

Their quest for thrills sweeps them up in a frenzy of creativity and courage. From conceiving a homemade roller coaster in the backyard to embarking on a mythical Bigfoot hunt in the local woods, every challenge propels them into an adventure more outrageous than the last. Their greatest feat yet is a city-wide treasure hunt, a course brimming with clues and excitement that stretches their imaginations and their friendships.

Despite the elaborate blueprints Judy and her gang devise, it is the unforeseen twists and spontaneous escapades that truly define their summer. Each thrill attained is another story, another memory, unfolding amidst laughter and mishaps. Judy’s journey unravels along the way, revealing that the essence of summer fun isn’t in the plan, but in the surprises and the people who share them.

With each thrill-seeking endeavor, Judy discovers profound lessons wrapped in joy and camaraderie. Bound by the ties of friendship and the warm embrace of family, she realizes that the heart of an unforgettable summer is not solely in achieving thrills but in embracing the unpredictability of life. It’s the unplanned detours and the companionship of those nearest and dearest that transform a "not bummer summer" into the adventure of a lifetime.