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The story follows a reclusive mathematical genius, Kim Seok-go (played by Ryu Seung-beom), who lives a quiet life as a high school math teacher. He secretly harbors feelings for his neighbor, Hwa-sun (played by Lee Yo-won), a divorced single mother running a small lunch box shop.

One night, Hwa-sun’s violent ex-husband breaks into her apartment to cause trouble. In a desperate struggle, Hwa-sun and her daughter kill him in self-defense. When Seok-go discovers the body, he doesn’t call the police. Instead, his mathematical mind kicks into overdrive. He devises a "perfect" crime—a flawless alibi involving complex equations, logical fallacies, and misdirection.

The "hot" part of the drama arrives when Detective Min-bum (played by Jo Jin-woong), a brilliant but unorthodox investigator and Seok-go’s former college rival, takes the case. What follows is a chess match of intellects. Can a perfect logical solution withstand the messy, unpredictable nature of human emotion?

Perfect Number (Korean title: Yong-ui-ja X – literally "Suspect X") is a 2012 South Korean mystery drama directed by Bang Eun-jin. The film is an adaptation of the famous Japanese novel The Devotion of Suspect X by Keigo Higashino. (Note: A famous Japanese adaptation exists from 2008, but the 2012 Korean version is distinct).

Why is it called "Perfect Number"? In mathematics, a perfect number is a positive integer that equals the sum of its proper divisors (e.g., 6 = 1+2+3). In the film, the title is a metaphor for the protagonist’s attempt to construct a logically perfect, unbreakable alibi—a mathematical solution to an emotional crime.