Yoona's mum died during childbirth and she was left in her dad's care. Yoona grew up with the qualities of her mother. Always cheerful and always positive. However, behind her cheerful smile and positive attitude, she yearns for a mother's love. Now, 18 years later, her dad (Im Sung Jo) proposed to the woman he met and dated a year ago, Kim Dae Young, who has 12 sons. Yoona and her dad moved in to Dae Young and her sons' mansion and prepares for the wedding.

Yoona's world turns upside down as she gains 12 handsome brothers.

The 12 brothers' world turns upside down as they gain an adorable little sister.

Will they acknowledge Yoona as their adorable new sibling or will they entangle themselves in a love triangle and be each others' rival?

Will they choose their parents' happiness or will they choose to fight for their love towards their little sister?