I Was His First Mistake is a high-IQ revenge short drama set in New York's upper-class society. Quantitative analyst Ivy Chris is framed by her fiancé Derek Kane and sent to prison for five years. After her release, she is hunted down and saved by Julian Cross, a mysterious capital manipulator. The two enter into a contractual marriage, and Ivy returns to the Manhattan social scene as Mrs. Cross, using financial tactics and art-world deceptions to exact revenge on the Kane family. Yet amid the cold calculus of power plays, Ivy gradually begins to suspect that the man who saved her may be the deepest trap of all. The series unfolds at a breakneck pace—one explosive twist per episode, a reversal every two episodes—weaving together power, desire, and intergenerational reckoning.