Contains spoilers for this episode and earlier episodes.
Stan announces a family outing and takes Dipper and Mabel to Lake Gravity Falls for a day of fishing. The twins dread hours of knots, worms and old jokes, even after Stan gives them fishing hats with their names on them. Old Man McGucket interrupts the gathering with warnings about a lake monster called the Gobblewonker. Dipper sees a chance to photograph the creature and win a cash prize. He and Mabel abandon Stan's boat to search with Soos, who brings them to Scuttlebutt Island. Dipper supplies seventeen disposable cameras, convinced that preparation will prevent the usual blurry monster photographs.
The hunt produces false alarms, wasted cameras and a colony of beavers before a huge creature finally appears. It pursues the group across the island and onto the lake, turning their search for a picture into an escape. They lure it into a narrow cave, where damage exposes machinery inside its body. McGucket built the mechanical Gobblewonker because his son would not spend time with him. Hearing his explanation makes the twins reconsider leaving Stan alone. They return to their great uncle and spend the rest of the outing enjoying his company, taking photographs of their time together. Beneath the water, beyond the family's knowledge, a living Gobblewonker passes through the lake.