One chance encounter, he found her crying on the street.
Her sister had a car accident and the driver escaped. Now her sister has to spend a lot of money for surgery, but she has no money. Her parents left early, so she and her sister are the only ones relying on each other.
After hearing her story, he decided to make a deal with her.
He paid her to become his "little contract bride", and she was eager to save her sister, so she agreed to his unreasonable deal.
The day after he got the money, she mysteriously disappeared. He decided to catch his own unfaithful little contract bride at all costs.
She must be severely punished for deceiving him.