The battle served as a diversion for Queen Amidala, who planned to strike at Federation-controlled Theed, and needed the main Federation force drawn away from the city. Viceroy Nute Gunray ordered the main army away from Theed to deal with the threat, leaving a security force that was minuscule by comparison to defend Theed.
The Gungans fought for several hours; however once their energy shield was breached they were swiftly routed by OOM-9's forces in a tactical defeat. However, the destruction of the Droid Control Ship by Anakin Skywalker caused the Federation army to deactivate, which resulted in a final victory for the Gungan army.
The Queen implored the Galactic Senate to assist her against the Trade Federation's illegal occupation of her home planet of Naboo, but could not bear the suffering of her people while the Senate deliberated.
Deciding to take action herself, she returned to Naboo and, with the aid of Jar Jar Binks, the Queen managed to convince Rugor Nass, leader of the Gungans, to align with the Human population of Naboo. Nass pledged his support to Amidala, and a historic pact was made, ending years of enmity between the Naboo and the Gungans. Once the remainder of the Royal Naboo Security Forces had been summoned to their hiding place, the Queen, Captain Panaka, and Nass devised a plan to retake Naboo.
While this was going on, Nass made Binks a Bombad General for this oncoming battle. The Gungan Grand Army were to lure out the main bulk of the Federation's Droid Army onto the Grassy Plains for an enormous head-to-head assault, allowing Amidala and her strike force, along with the two Jedi, Qui-Gon Jinn and Obi-Wan Kenobi, to attack Theed and liberate the Theed Hangar. Naboo pilots would then attack the Droid Control Ship, deactivating the Federation Army, and allowing Amidala to capture the Viceroy, thus re-gaining control of the planet.
Nass, however, was actually without an organized army. He took what troops he had and destroyed a Federation monument that had been erected in the city of Spinnaker. In its place, he had a temporary Gungan monument built, which he would use to rally and summon all of the militiagungs that had been in hiding around the town. With his army in place, all he needed was the weapons with which to equip his army. During the Naboo Swamp Battle, OOM-9 had captured the Gungans' fambaas, which were their beasts of war. Nass attacked the city of Harte Secur, where the fambaas were being held, and rescued them from captivity. With all of the pieces in place, he moved the Grand Army out of the swamps and onto the Grassy Plains.
The Gungan Grand Army proceeded to move out of its hiding place in the Swamps; they had licked the wounds of the Naboo Swamp Battle and were ready to take back what was theirs. The Federation's actions during the previous battle reflected little more than a massacre. Many civilians were killed during the evacuation of Otoh Gunga, leaving many militiagungs without a family, and without a home.
They moved into their position, taking up much of the level part of the Grassy Plains, and activated their shield generators, set upon the backs of the giant fambaas. The army was encased in an enormous dome, impervious to any bombardment. OOM-9's forces, fresh out of Theed, descended from the hills, in a uniform formation of Armored Assault Tanks and Multi-Troop Transports, however, much to the Gungan forces' surprise, with no ground troops present. They settled into position, facing the massive Grand Army. OOM-9 commenced preliminaries, firing on the Gungans' shields with his AATs, but to little effect.
After a while, OOM-9 ordered the AATs to hold fire, and, with an order from the Droid Control Ship, the Federation's MTTs unloaded thousands of battle droids, much to the horror of the Gungans. The B1 Battle Droids slowly marched through the Gungan shield and commenced fire on the unprotected militiagungs. The Gungans, led by General Ceel and General Binks, fought with personal shields, catapults, Boomas, and cestas but they could not halt the persistent droid march. A squad of droidekas rolled onto the bloodied battleground, further increasing Gungan casualties.
Army A—the main force—kept up the frontal pressure on the main Gungan array, while armies B and C prepared to move in. Army B's approach was halted thanks to the efforts of Nym in his starfighter, the Havoc, and likewise, army C's efforts were halted by the Royal Security Forces.
Ultimately, the shield generator was destroyed by concentrated Droideka fire, deactivating the protective dome. OOM-9 immediately called upon all AATs to advance. Tank shells now ripped unobstructed into the Gungan lines, tearing up the previously untouched Grassy Plains. While the Gungan catapults had a large area of effect, they had a slow firing rate, and they were nearly useless when in motion. Gungan ranks fell to the mechanized Trade Federation Army. General Ceel shortly ordered a retreat and Captain Tarpals was barely able to save General Binks from death. In the end, both were captured, their army defeated.
Despite a crushing defeat for the Gungans, events altered dramatically when the Droid Control Ship over the planet was destroyed later in the Naboo Space Battle by Anakin Skywalker, deactivating the droid army, and leaving the Gungan survivors instantly liberated from their recent prisoner-of-war positions. They watched awestruck as their droid captors and enemy vehicles all around them suddenly shut down, realizing that while their own battle had been lost, the battle above them must have been won. They promptly celebrated by tipping over the deactivated droids.
Thanks to the valiant actions of the Gungans, Amidala succeeded in her quest to capture the Viceroy, and control of Naboo was returned to its rightful owners. Following the Gungan-Naboo armistice, Boss Rugor Nass had a Gungan colony created on Ohma-D'un, and he kept relationships with the Naboo friendly.
The battle was ported into a holopad historical mission by the Trade Federation, to show how the battle would have evolved if the control ship had not been destroyed, which included Darth Maul arriving later in the battle to assist the droid army.
The Battle of Grassy Plains first appeared in George Lucas's Star Wars Episode I: The Phantom Menace. Like the Battle of Endor that featured in Richard Marquand's Star Wars Episode VI: Return of the Jedi, Grassy Plains is inspired by the Vietnam War, in particular, the concept of a technologically inferior race besting a more advanced culture in battle. However, unlike Endor, Grassy Plains is not won by the protagonists, and they are only saved due to luck, and outside intervention.
An alternate non-canon version of the battle appears in Star Wars: Galactic Battlegrounds as a "what-if" bonus scenario. In it, player, controlling OOM-9's army is able to decimate Ceel's forces with the optional aid of Darth Maul, and push them back to a Gungan city, which is in turn, destroyed.