The earthbender captain served in the Earth Kingdom military during General Iroh's siege of Ba Sing Se, and had a strong dislike of Iroh and the Fire Nation from that point on.
Not long before the winter solstice of 99 ASC, he and his unit of earthbendingsoldiers stumbled upon a bathing Iroh and captured the retired general by trapping him in the spring with their earthbending. When a soldier commented that Iroh was a Fire Nation soldier based on his uniform, the earthbender captain quickly identified Iroh for whom he truly was: the Fire Lord's brother, the Dragon of the West, the "once great" General Iroh, though he smugly finished off by stating that Iroh was now their prisoner.
The captain and his unit set out immediately, riding through the night on their ostrich horses, to take Iroh to Ba Sing Se to face justice. Still angered over Iroh's attempt to conquer the city, the captain stated that the city was greater than Iroh, as it would not yield to him. Iroh acknowledged his defeat, mentioning that he was tired then, just as he was still tired now, and let himself purposely fall off his mount in order to leave behind his sandal for PrinceZuko to track him down by, unbeknownst to the captain, who payed no attention to Iroh's antics.
The next day, the captain asked Iroh what was wrong, when the latter had gasped in shock when he saw Aang's spirit pass by on Fang. Iroh initially stated that everything was fine, though swiftly reconsidered and complained that his handcuffs were too loose, asking for them to be tightened. Though he was surprised by such a request, the captain ordered his corporal to do as asked. However, Iroh had tricked him, as the firebender had heated his chains and pressed the corporal's hand against them, burning him, before firebending from his feet at the rest of the troops in order to facilitate his escape. While the earthbender captain attempted to calm down two started ostrich horses by pulling tightly on their reins, he watched as Iroh rolled down a mountainside.
The captain used his earthbending to send an avalanche after the ex-general, effectively halting his escape by trapping him underneath the sand. Together with two other soldiers, the earthbender captain slid down the slope. Having recaptured Iroh, the captain agree with his corporal that it was too dangerous to transport Iroh to Ba Sing Se, and concluded that the firebender needed to be dealt with immediately and severely.
At twilight, the Earth Kingdom unit had tied Iroh down to a rock with metal shackles, his hands spread out on the surface. The earthbender captain noted that Iroh's hands, capable of great danger, needed to be crushed, and proceeded to lift a boulder with his earthbending. However, the execution of the punishment was interrupted by Zuko who kicked the rock away and freed his uncle. The captain demanded their surrender, stating that they were clearly outnumbered, to which Iroh cleverly responded that although that was true, the captain and his men were clearly outmatched.
A fierce duel ensued, in which the captain held his own, showcasing his proficiency in earthbending by launching boulder after boulder. After a short time, the captain was the only one left standing, but he managed to keep Zuko at bay. He raised the earth high above him, intending to crush the Fire Nation prince under a mixture of sand and rocks in a finishing move. However, before he could execute the attack, he was immobilized by Iroh, who entangled a chain around the earthbender captain's ankles and yanked his feet from underneath him. As the captain fell down, he lost the control of the raised landmass and the rocks crashed down on top of him, effectively knocking him out cold. He was spared by the two firebenders, as they left to continue their own journey.