The story begins with the Belcher family facing the imminent foreclosure of their burger restaurant after failing to secure an extension on their loan. Bob and Linda are determined to save their struggling business, but things take a turn for the worse when a massive sinkhole appears right in front of the restaurant's entrance. As they scramble to devise a plan to pay off their debts, the sinkhole reveals a hidden tunnel that piques the curiosity of their three adventurous children—Tina, Gene, and Louise. Hoping to discover something that could salvage their family's livelihood, the kids embark on a journey into the mysterious underground passage.

As the Belcher children delve deeper underground, they encounter clues that unravel a long-standing mystery tied to the town and its inhabitants. Meanwhile, Bob and Linda devise unique strategies, including hosting extravagant burgers taste tests and engaging with quirky community members, to attract customers despite the obstructive sinkhole. Both endeavors run simultaneously as the family races against time. Their separate quests lead to unexpected connections between events in the town's past and present day challenges. The plot intertwines humor, suspense, and heartwarming moments as each family member learns valuable lessons about resilience, community, and the enduring importance of family bonds while trying to reclaim their beloved restaurant.