Tahn was an Earth Kingdom refugee who sought shelter in Ba Sing Se with his family during the closing years of the Hundred Year War. A jovial man who was always happy to greet fellow refugees, Tahn was a caring husband with a level head and always tried to have an optimistic outlook on life, no matter how bleak it seemed. He would provide comfort and words of hope for his wife when she became discouraged.
Tahn lived in a village with his sister before meeting Ying, with whom he fell in love, married, and began a family. However, as Ying was pregnant with the couple's first child, the Rough Rhinos invaded their village, forcing Tahn and his family to flee to Ba Sing Se to start a safer life there. They traveled for months up the west coast.
Tahn and his family were briefly seen by Zuko when he was traveling alone. Tahn was catering to his pregnant wife and the exiled, starving prince considered stealing from them, but ultimately decided against it upon seeing that Ying was pregnant.
Weeks later, the family met Aang and his friends by a waterfall when they too planned to journey onward to Ba Sing Se. Katara invited them to travel with them through the Serpent's Pass, but Ying informed them that the pass was very dangerous in comparison with the ferry system at Full Moon Bay, their destination. Subsequently, Team Avatar joined Tahn and his family in their journey to the hidden bay.
At the bay, Tahn and his family split ways with Team Avatar, though they sought their help after someone stole all their belongings, including their passports and tickets. Aang tried to persuade the clerk to replace their tickets, but she refused, stating that she could not bend the rules. With the family out of options, Team Avatar gave up their own tickets and offered to guide the family through the Serpent's Pass.
The pass was very straight, although narrow, forcing the group 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, but not before the falling rocks drew the attention of a patrolling Fire Nation ship, which launched a fireball at them, forcing them all to take shelter behind an outcropping wall of rock. Getting through the rest of the day without more trouble, Tahn helped Ying to recover from the travel by giving her a relaxing foot massage.
The next morning, the travelers were faced with a submerged portion of the way. Katara and Aang parted the waters, forming a bubble of air allowing for them to walk underwater safely. However, the serpent for which the pass was named attacked the bubble, forcing Toph to raise the sea-floor under their feet, stranding them in the middle of the water. Katara created a makeshift passageway of ice to other side while Aang distracted the animal, enabling Tahn, who protectively held and supported his pregnant wife, and the others to safely cross the ice bridge.
Upon exiting the Serpent's Pass, Ying went into labor. Tahn supported her once again, lovingly holding her hand, while Katara, who had experience in delivering children, quickly took charge. After the baby was born, Tahn agreed to name their daughter Hope, proclaiming that it was "a perfect name", in reference to how the entire ordeal had helped Aang through his own emotional turmoil.
When Aang discovered that the Fire Nation was about the penetrate Ba Sing Se's defenses 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. The drill was eventually destroyed, after which Tahn and his family ventured into the city via 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 the streets of the city sometime after Princess Azula's successful coup, Tahn protectively clutched his distressed family.