Ant Island is a colony of ants led by the Queen and her daughter, Princess Atta. Every season, they are forced to give food to a gang of marauding grasshoppers led by Hopper. One day, when Flik, an individualist and would-be inventor, inadvertently knocks the offering into a stream with his latest invention, a grain harvesting device, Hopper demands twice as much food as compensation. When Flik suggests in earnest that they seek help from other stronger bugs, the other ants see it as an opportunity to remove him and send him off.
At the “bug city”, which is a heap of trash under a trailer, Flik mistakes a troupe of Circus Bugs (who were recently dismissed by their greedy ringmaster, P.T. Flea) for the warrior bugs he seeks. The bugs, in turn, mistake Flik for a talent agent, and accept his offer to travel with him back to Ant Island. During a welcome ceremony upon their arrival, the Circus Bugs and Flik both discover their mutual misunderstandings. The Circus Bugs attempt to leave, but are attacked by a bird; while fleeing, they save Dot, Atta’s younger sister, gaining the ants’ respect in the process. At Flik’s request, they continue the ruse of being “warriors”, so the troupe can continue to enjoy the hospitality of the ants. Hearing that Hopper fears birds inspires Flik to create a false bird to scare away the grasshoppers. Meanwhile, Hopper tells his gang how greatly the ants outnumber them and worries that they will eventually rebel against them.
The ants finish constructing the fake bird, but during a celebration, P.T. Flea arrives, searching for his troupe, and he inadvertently reveals their secret. Outraged by Flik’s deception, the ants exile him, and desperately attempt to gather food for a new offering to the grasshoppers. However, when Hopper returns to discover the mediocre offering, he takes over the island, and demands the ants’ winter food supply, planning to assassinate the Queen afterwards. Overhearing the plan, Dot informs Flik and the Circus Bugs, convincing them to return to Ant Island.