Friends Cowboy, Indian, and Horse share a tranquil life in their rural home. However, things take a turn when Cowboy and Indian unexpectedly forget Horse's birthday. In an effort to keep their oversight under wraps, they devise a plan to secretly construct a brick barbecue for him. They persuade their neighbor Steven to send Horse on a mission to pick up his animals from the nearby music school, thus buying themselves some time.
While at the music school, Horse encounters his romantic interest, Mrs. Jacqueline Longray, a charming fellow horse and music teacher. He is smitten and eager to impress her, even attempting to play the piano to win her over. To his delight, she offers to give him lessons, sparking a connection between them.
Back at their home, chaos ensues as Indian attempts to order fifty bricks for the barbecue but ends up mistakenly ordering fifty million instead. To manage the surplus, Cowboy and Indian stack the extra bricks into a cube and place it atop their house before finally completing the barbecue project. However, disaster strikes that night as the weight of the bricks causes the house to collapse completely. Furious, Horse demands they rebuild it.
As they begin the process, an unseen culprit starts stealing the newly constructed walls. After a stakeout, the trio discovers that the walls are being pilfered by a family of cone-headed aquatic creatures. Despite their efforts, most of the culprits escape with the walls, leaving only one behind, named Gerard. In a comical chase, they pursue Gerard, ultimately tumbling into the Earth's core.
When they emerge, they find themselves in a frigid tundra. As they explore their new surroundings, they are unexpectedly sucked into a giant robot penguin operated by strange, powerful scientists. They come face-to-face with Gerard again, but this time, they all get captured and forced to work for the scientists. In the midst of a chaotic confrontation with a rogue mammoth, the group manages to escape by hitching a ride on a launched snowball aimed at their home, albeit Gerard redirects it to his underwater habitat.
Following Gerard, they don scuba gear—though Cowboy chooses to get creative by donning a TV on his head—and swim after him. They discover an underwater version of their house, realizing that Gerard and his family intended to use the walls for constructing their own home. The aquatic creatures attempt to chase them away, but the friends cleverly turn the tables by having Horse disguise himself as Santa Claus, tricking the creatures. They ultimately use a sawfish to demolish the creature’s house, making a daring escape to the surface. However, the aquatic family pursues them with swordfish attacks. Luckily, Steven, his wife, and their animals come to their aid. The ensuing battle leads to a dramatic explosion of water that floods the countryside.
Fast forward a year later, Gerard's family has become integrated into the community, and Horse has flourished into a talented piano player, now dating Mrs. Jacqueline Longray. To commemorate the occasion, she surprises him with a birthday party in an underwater department store. In a hilarious twist, Cowboy and Indian inadvertently ignite Horse's birthday firework gift, resulting in a colossal fireworks display that obliterates the surrounding landscape just before the credits roll.
