"Jiisan Baasan Wakagaeru" centers on an elderly couple, Shouzou and Ine Saito, who unexpectedly regain their youth after a mysterious incident. Shouzou, a 78-year-old man, and Ine, his 74-year-old wife, wake up one morning to find themselves transformed into their 18-year-old selves. Despite their youthful appearances, they retain the wisdom, experiences, and personalities of their older selves. The series explores their daily lives as they navigate the challenges and opportunities of being young again while maintaining their long-standing relationship and values.

The narrative delves into their interactions with family, friends, and society. Their adult son, Kiyoshi, and his family are initially shocked by the transformation but gradually adapt to the situation. The couple’s newfound youth allows them to experience activities and opportunities they missed in their original youth, such as pursuing hobbies, reconnecting with old friends, and even considering part-time jobs. However, their mature perspectives often lead to humorous and heartwarming contrasts with the behavior of actual teenagers around them.

Throughout the series, Shouzou and Ine face dilemmas that arise from their dual identities. They must balance their desire to enjoy their second youth with their responsibilities as elders in their community. The story also touches on themes of aging, relationships, and the value of life experiences, as the couple reflects on their past while embracing their present. Their journey is marked by lighthearted moments, emotional introspection, and the enduring bond they share, showcasing how their love and partnership remain constant despite their physical transformation.