Located in the middle of a riverbed, Ant Island is somewhat protected from outside threats, except for flying ones. The riverbed floods every autumn, but during the summer, it is quite dry. There is a tree in the middle of the island, and right at its base is the anthill. There is seemingly enough food for both the ants and grasshoppers; however, when Flik accidentally destroys the seasonal offering for the grasshoppers, their leader Hopper demands twice the amount of food, and it is revealed that there is not enough for them and the ants.
After Flik is banished from the island for deceiving the colony, when P.T. Flea came looking for his troupe and blew Flik's cover in doing so, the grasshoppers take over the island and hold the colony hostage. Flik later decides to return after Dot (who had just learned to fly) shows up and informs Flik that Hopper and his gang are eating all the food on the island and plan to kill the Queen when they are finished. Flik and the circus bugs then hijack P.T. Flea's circus wagon and head back to the island.
Upon arriving, Flik, Dot and the Blueberries use a fake bird to scare Hopper and his gang, but it is set on fire by P.T. Flea who mistook it for a real bird and accidentally aided Hopper in the process. Hopper then has Thumper beat Flik up after he reveals that he was the one who came up with the hoax. However, the colony rebels against Hopper, but it soon starts to rain. Hopper kidnaps Flik and attempts to fly away with him, but he is rescued by Atta and they lure Hopper into the real bird. Hopper mistakes it for a fake bird and taunts it, causing her to shriek angrily at him. Hopper realizes it is a real bird after all and she feeds him to her chicks, thereby putting Hopper's reign of terror and insult on the ant colony to an end, and the ants get their island back.
The Battle for Ant Island
With the circus bugs agreeing to help the ants, they all begin work on a fake bird which should scare away Hopper and his gang for good. However, P.T. Flea arrives on the island and reveals that the new bugs are just circus bugs and not trained warriors. Distraught, Princess Atta banishes them along with Flik to go back to where they came from. However this was a fatal mistake to Princess Atta, she ends up putting herself and the entire colony in danger and the Queen's downfall when Hopper and his gang take control of the island.
Dot catches up with the circus wagon, and soon persuades Flik and the others to return and help fight. Before they can do so, they tie up P.T. (using Rosie's web) and lock him in the back of the circus wagon. Flik, Dot and the circus troupe watch from a distance as the grasshoppers take the ants hostage and take all of the food. They decide that their first priority is to save the Queen, who is sitting next to Hopper.
Flik and Dot enter Dot's hideout and regroup with the scout ants. Meanwhile, the Circus Bugs entertain the grasshoppers, and put on a show. They lie that they were invited by Princess Atta as a surprise for the grasshoppers' arrival. At first, Hopper orders them to be squished. But after seeing Tuck and Roll's act, he decides that he may enjoy the entertainment. While the grasshoppers are distracted, Flik and the young scouts climb up the rocks behind the enemy and enter the fake bird they had made earlier.
During the circus acts, Manny asks for someone to join him as an assistant so he can make them disappear. With Hopper's permission, Manny takes the Queen on "stage" with him. In actual fact, the Queen has escaped under the wings of Dim. When Manny lifts the box and the grasshoppers see that the Queen has disappeared, they accept it as part of the magic act. Gypsy flaps her wings and bows with Manny as the crowd cheer her, but it is really a signal for Flik to fly down with the fake bird. Hopper soon grows irritable when Manny fails to bring the Queen back, and the grasshoppers close in on the circus bugs. However, they all look up in horror as the fake bird (unknown to them, driven by Flik) swarms over them.Taking their chance, the hostage ants run. Seeing the bird, most of the grasshoppers retreat and leave the island. The bird accidentally knocks over the circus wagon, and P.T. manages to escape. Seeing the bird, he too believes it to be real. Thinking he is being a hero, he grabs a match and sets the bird alight ending up aiding Hopper and his gang. Seeing it burn, Hopper realizes it is a fake while Flik and the girl scouts, inside, escape the fire. Hopper, now furious and embarrassed, asks whose idea it was. Flik owns up, and is beaten viciously by Thumper. Despite his beatings, Flik stands up and points out it's not the ants who need the grasshoppers, it's the grasshoppers who need the ants. Realizing he is right, the ants link arms and know they can outnumber Hopper's gang. Thumper, Molt and the remaining grasshoppers flee.
The ants attack Hopper, and plunge him into the circus cannon. Just as they are ready to fire him as far away from Ant Island as possible, it begins to rain. The rain knocks over the cannon, and Hopper escapes. He then flies off with Flik. Francis, Slim, Tuck and Roll, Rosie, Dim and Heimlich take chase. Tuck and Roll manage to pull off one of Hopper's antennae, but the circus bugs soon get lodged in a group of branches. Princess Atta flies past, and snatches Flik away from Hopper. Flik and Princess Atta lead the grasshopper to the real bird's nest, where the ants hide. The real bird suddenly appears, but Hopper thinks it is another one of the ant's tricks. To his horror, it is a real bird who picks him up and drops him into a nest of three hungry chicks, thus ending Hopper's reign of terror and insult on the ant colony. The grasshoppers are defeated, and the ants have their island back.
- During the "When She Loved Me" musical sequence in Toy Story 2, Emily takes Jessie to a tree in the summer, where they swing on a tire. When Emily grows up, it is this same tree that Jessie is left at. This is the same tree as the one in Ant Island.