Ji Tingfu and Fu Sizhuo are childhood sweethearts and fire-and-water arch-enemies. The sudden appearance of a five-year-old boy, Fu Niannian, sweetly calling them mom and dad forces the two to team up to hide the child and start a "cohabiting child-rearing" mode. Amidst chaotic daily routines such as amusement park parent-child competitions, sharing a bed on thunderstorm nights, and meeting through the window, they gradually discover each other's feelings hidden for many years. Meanwhile, Cheng Xi, impersonating the child's birth mother, breaks into the Fu family with a forged paternity test, attempting to seize the opportunity to marry into high society. Suddenly, Niannian time-travels and disappears, plunging Ji Tingfu and Fu Sizhuo into a time-space paradox.