On his 25th birthday, Li Nianyu unexpectedly time-travels back to 1996, becoming that mysterious old acquaintance named "Gu Yu" in his parents' youth, while the 90-day life countdown on his wrist quietly starts. As his mother's male best friend and his father's sworn brother, he accompanies them through an entire summer, being treated as a love rival by his father, sharing funny daily routines living with him, helping his clumsy father pursue love, bailing out his mother, and making up for all the regrets in their youth. When the countdown hits zero, he returns to 2026, finally understanding that the "Nianyu" in his name hides his parents' longing across thirty years. It turns out that all encounters are truly reunions after a long separation.