My Young Auntie tells the story of Cheng Tai-Nan (Kara Hui), an honest and faithful servant of a dying patriarch who is determined to protect his vast wealth from his selfish nephew, Yung-Sheng.
Tai-Nan, who is young enough to be his granddaughter, agrees to marry her master in order to safeguard his wealth and transfer it to Ching-Chuen (Lau Kar-Leung), her new husband's favorite nephew.
This decision angers Yung-Sheng, who sends thugs after Tai-Nan. However, she proves to be a highly skilled martial artist who cannot be easily defeated.
Things become even more complicated when Ching-Chuen's son, Yu Tao (Hou Hsiao), returns home from university in Hong Kong and unknowingly attacks Tai-Nan, mistaking her for a mysterious woman in his house.
Adding to the complexity is the uncomfortable sexual tension between Yu Tao and Tai-Nan. Eventually, Yung-Sheng manages to steal all the paperwork and titles to Ching-Chuen's wealth.
Left with no choice, Tai-Nan leads an all-out attack against Yung-Sheng at his booby-trapped mansion, with the goal of getting Ching-Chuen and his brothers back into fighting shape.