After finishing the open examination for Form 5 students and applying for an adult identity card, Yoyo, eighteen, departs for London to meet a guy introduced by her well-meaning parents in Britain. The guy, named Cheung, is in his thirties. Yoyo's and Cheung's grandfathers were wartime buddies. They swore to have their unborn babies get married when they grew up. However, the babies turned out to be both women. The two women are now mothers. They come across each other again and want to make their fathers' wish come true, so they arrange for Yoyo and Cheung to meet in London. Yoyo and Cheung promise to get marry in order to please their mothers. But on the wedding day, they also sign a divorce agreement which is effective one year after their marriage.

My Sassy Husband follows Yoyo Ma and Thirteen Cheung’s union 10 years later to present day where they are experiencing marital problems. After they realize that their wedding ceremony held in London is not considered official in Hong Kong, they decide to part ways in search of their real love. While Yoyo and Thirteen both start dating new prospects, will they end up back together?