It's actually based on a novel and I was able to find a summary online with the help of google translate. So here's another synopsis.
Kangsadan is a dreamer who used to view the world in an positive light until her pink colored world changed when she became the woman of Punsakorn, the only man she loves. He made her an offer for her to become his mistress in exchange for the house, the only memory she has left of her mother. Punsakorn is the only heir of a millionaire/billionaire who hides his cruel coldness under his good looks. He says he never has eyes for Kangsadan, the little girl who fell in love with him. But one day his heart began to waver.
Kangsadan stepped up to the status as the woman his father loves. (More like his father adores her like she is his own daughter kind-of-way.) With resentment in his heart, Punsakorn decided to turn his back on his family. After four years, he came back as a charming and handsome businessman to become business rivals with his father's company. He returned in order to take everything back, and Kangsadan will be the first person he takes revenge on.