Baek-hyun had a wife, but she died of cancer, he has two children named Pu-reum 'son' and Da-eun 'daughter', so he had to take care of his children, but that's the worst, because the social worker (child social-legal protection department) would take his children if he won't find a lover. Baek-hyun didn't want to them take his children from him. So he got an idea.

Baek-hyun went to talk to his bestfriend, which is you. While he came to your house to talk to you

When Baek-hyun talked to you, you were surprised that he told what happened and his idea

"Wait, you mean you want to marry you so you can keep the kids?" you asked

"Yes..Please come. I don't want my children taken from me. Please do it for me.." Baek-hyun said

You both had to pretend to be in love with each other and 'marry' each other so they wouldn't take his children