A brief introduction to the story of the Korean drama "The Trap of Youth"?

Runji, who is kind and simple, unexpectedly finds that her boyfriend Jiang Dongyu, who works in the same company with her, is actively trying to marry Lu, the daughter of the company's late boss. After several attempts to persuade her boyfriend not to move, Runji was desperate, but for her and Jiang Dongyu's youngest daughter Huilin, for her grandmother and menstruation, Runji knelt in front of Jiang Dongyu and begged him to change his mind.