Since the beginning of the War, the Fire Nation had searched for a way to conquer the capital of the Earth Kingdom, Ba Sing Se; however, all previous attempts had failed. The country hoped that, by conquering the city, the Earth Kingdom would be severely crippled once the Council of Five and the Earth King were taken out of power. A successful subjugation also meant conquering the last safe place for refugees displaced by the War. Late in the War, the Fire Nation forced a refugee and Earth Kingdom mechanist to make weapons, including the drill, to aid them in battles by threatening to burn his home if he refused.
The mechanist successfully designed the drill, a gigantic mechanism that was nearly impossible to destroy and could breach the walls of Ba Sing Se with ease. The Fire Nation hoped that by breaching the walls, they would be able to set a path for their army to advance and finally conquer the great city.
After completion, the drill and a small force of tundra tanks headed toward the great city to break in. The force arrived at the exact time as Team Avatar spotted the device.
Ba Sing Se's army was unprepared due to never having fought a major battle in years, and their only means of defense was throwing boulders at the drill, which had little success. General Sung, who commanded the troops on the Outer Wall, did not see the drill as a real threat and was confident that the Terra Team, a group of elite earthbenders would stop the colossal machine. Despite their best efforts, they were no match for Ty Lee and Mai, who promptly ambushed the soldiers. Utilizing chi blocking, Ty Lee disabled the entire team within mere moments. With the Terra Team helpless, Sung panicked and pleaded with Team Avatar for help despite his earlier thoughts of not needing it at all.
Team Avatar came up with a plan of hitting the drill's pressure points as Ty Lee had done to defeat the Terra Team. Aang, Katara, and Sokka infiltrated the drill while Toph tried vainly to slow it down from the outside because she had not learned to metalbend yet, and thus could not 'see' inside the drill. Having stolen a schematic from a drill worker, the group headed for the outer-most part of the drill where massive metal braces held the entire mechanism together. Aang and Katara began cutting completely through the braces using waterbending, but found the process was too slow to successfully disable the drill before the it broke through the wall. Aang stated that they did not need to cut through the braces completely and that just halfway through would be sufficient. Following this, he could deliver a final blow from the exterior that would destroy the weakened braces. Meanwhile, the crew found the drill worker that Katara had frozen earlier, prompting Azula to investigate the situation along with Mai and Ty Lee. The gang fled as they could not fight and cut through the braces at the same time.Aang eventually separated from Sokka and Katara to deliver the final blow on top of the drill. Katara and Sokka managed to escape the machine by riding the slurry, a mixture of water and earth used to deposit the rock from the wall. Ty Lee followed them into the pipe, but Mai did not, stating "Azula can shoot all the lightning she wants at me, I'm not going in that wall sludge juice." Because the slurry was water and earth combined, both Katara and Toph were able to bend it from the outside and build up more pressure for when the final blow was delivered. Ty Lee became trapped in the slurry since Katara and Toph were bending it back into the inside of the drill.
Meanwhile, Aang dodged falling boulders from Sung's men as the general thought that this would be an effective measure, even though it clearly was not. He also fended off Azula as he cut a hole into the metal exterior using waterbending. Initially, Azula maintained the upper hand, dodging all of Aang's attacks, be it earthbending, waterbending, or airbending. Eventually, she launched a massive fire blast that knocked Aang unconscious. As she prepared to deliver the fatal blow, Aang recovered and fought her off with earthbending. After removing her from the fight, Aang resumed his destruction of the drill, utilizing a boulder to his advantage to make a wedge shape which he shoved into the indent he made earlier. He subsequently used airbending to speed-run steep down the Outer Wall, gaining momentum for the final blow. Azula launched a last-ditch attack to save the drill, but this failed and Aang landed on the wedge with huge force, sending Azula flying off the drill.Aang's powerful attack caused the weakened braces to break apart simultaneously, and the drill to collapsed into a slurry mess, its pipelines exploding as the vehicle caved in. This caused the entire area to be flooded in slurry, and it buried a large number of the Fire Nation troops escorting the vehicle or working within. Ty Lee was ejected from the slurry system, but was largely unharmed. Azula also managed to escape injury. In the wreckage, a pristine and nonchalant Mai humbly said, "We lost."