Tahn was an Earth Kingdom refugee traveling to Ba Sing Se with his family. A jolly man who was always happy to greet "fellow refugees", Tahn was a caring husband with a level head who seemingly tried to have an optimistic outlook on things, no matter how bleak. He was always there to comfort and offer words of hope to his wife when she became discouraged.
After his home in the south was raided by the Rough Rhinos, Tahn began the trek to Ba Sing Se to start a safer life there for his family, consisting of his then-pregnant wife, Ying, and his sister. They traveled for months up the west coast.
Tahn and his family were briefly seen when Zuko was traveling alone. The exiled prince was starving and wanted to steal from them, but changed his mind when he observed that Ying was pregnant.
After briefly residing by a waterfall, Aang and his friends planned to journey onward when they ran into these Ba Sing Se-bound travelers. Katara invited them to travel with them, but Ying informed them that the Serpent's Pass was far too perilous. Instead, they planned on traveling via an underground ferry system at Full Moon Bay which led to Ba Sing Se. Aang and the others agreed to travel with them.
However, when they reached the station, they found the passport attendant to be very strict and ruthless. Tahn and the family sought Aang's help, after someone had stolen all their belongings, including their passports and tickets. Aang tried to persuade the clerk to give them replacements, but she refused, complaining that she could not bend the rules. With no other options for the family, Team Avatar gave up their own tickets and offered to guide the family through the Serpent's Pass.
The group reached the pass, which, seemingly contradicting its name, was very straight, although narrow, forcing people to pass through single file. As the group traveled along a sheer cliff side, Tahn lost his balance and fell when the ground beneath him crumbled. Toph's immediate reaction saved him; however, the falling rocks drew the attention of a patrolling Fire Nation ship, which launched a fireball at them. Aang leapt into action, redirecting the ball back with his airbending and striking the ship's engines. The group ran to the shelter of an outcropping wall of rock, leaving the ship behind to tend to its damage.
The next morning, Aang's group came to a large setback: a portion of the pass was completely underwater. Katara stepped forward, and with Aang's help, parted the waters to allow them to pass, forming a bubble of air for them to walk underwater safely. They found out, however, that the Serpent's Pass was named and feared not for its shape, but for the gigantic sea serpent that swam in the surrounding waters. The serpent attacked the bubble, forcing Toph to raise the sea-floor under their feet, stranding them in the middle of the water, with the serpent circling around. Aang attacked the serpent, telling Katara to get the others across while he distracted it. Tahn, protectively holding and supporting his pregnant wife, safely crossed Katara's ice bridge along with the other members of the group.
The group finally crossed the Serpent's Pass, just as Ying went into labor. Katara, who had experience in delivering children, quickly took charge, while Tahn supportingly held Ying's hand. When Aang admitted that he had been going through a hard time lately, but that the family had made him hopeful again, Tahn agreed with his wife to name their baby Hope, saying that it was "a perfect name".
When Aang discovered that the Fire Nation was about the penetrate the great wall of Ba Sing Se with a giant drill, Tahn and his family were brought to the top of the great wall when the Avatar and his friends decided to help out. After the drill was destroyed, Tahn and his family ventured into the city by using the Ba Sing Se Monorail, on which they encountered Iroh and Zuko, without knowing their true identity. Arriving in the Lower Ring, the family established a residence there.
When the Fire Nation troops swarmed through the streets of Ba Sing Se sometime later after Princess Azula's successful coup, Tahn was seen clutching his distressed family as the Fire Nation invaded the Earth Kingdom capital.