The cunning and inventive Qin oversaw the creation of all the Fire Nation's war machines and had several secret weapons in the works in an effort to bring an end to the Hundred Year War. Qin had been a Fire Nation emissary for years.
When the Fire Nation threatened to burn down the Northern Air Temple, the mechanist had offered his services to save the temple from destruction. He worked for Qin, who came to check on his inventions on a regular basis. This partnership led to the creation of destructive tanks and warships. When an impatient Qin later visited the Northern Air Temple, he was forced away by AvatarAang. Before Qin exited, he warned them that the Fire Nation would burn the temple to rubble. The Earth Kingdom refugees that lived in the temple put up a brave fight, and with the mechanist's latest invention, a hot air balloon, they managed to push the invaders back completely. During this battle, however, Sokka and the mechanist were forced to abandon the balloon which crashed in the nearby valley. Here, War Minister Qin discovered it and quickly learned its importance, stating that "this defeat [would be] the gateway to many victories." After the discovery of the hot air balloon, Qin turned it into a frequently used war balloon, enabling the Fire Nation to control the skies, as well as the land.
Months later, War Minister Qin revealed his masterpiece of technology that he had worked on for years: a massive drill developed for the penetration of the Great Walls of the "impenetrable city", Ba Sing Se. His plan was, however, foiled again by the Avatar and his friends.
After that technology's defeat, it became clear that Qin had been working to make more war balloons as well as adapting and improving the technology to create the larger Fire Nation airships.
During the invasion of the Fire Nation Capital on the Day of Black Sun, Qin was strolling down the halls of Fire Lord Ozai's secret bunker, where he came across Aang, Toph, and Sokka for a third time. When threatened by them, he quickly gave them directions to Ozai's chambers, where not Ozai but PrincessAzula awaited them. After this encounter, Qin resumed his stroll down the hall, whistling like nothing had occurred. When the eclipse ended, the fruition of Qin's engineering in the manufacture and improvement of war balloons was revealed as massive airships appeared and successfully bombed the invaders' submarines, preventing the majority of the strike force from escaping.