"I Hear the Sunspot: Theory of Happiness" is a sequel to the original manga "I Hear the Sunspot," continuing the story of Taichi Sagawa and Kohei Sugihara. Taichi, a cheerful and outgoing college student, meets Kohei, a reserved and partially deaf young man, in the first installment. Their relationship evolves from friendship to romance as they navigate the challenges of communication and understanding. In "Theory of Happiness," the narrative delves deeper into their lives as they face new obstacles and grow both individually and as a couple.

The story picks up with Taichi and Kohei adjusting to their relationship while dealing with the realities of Kohei's hearing impairment. Taichi, who initially struggled to understand Kohei's world, has become more attuned to his needs and has learned sign language to bridge the communication gap. However, their bond is tested when Kohei receives an opportunity to study abroad in France, a chance that could significantly advance his career but also separate him from Taichi. This decision forces both characters to confront their fears and insecurities about the future of their relationship.

Kohei's journey abroad introduces new dynamics, as he grapples with feelings of isolation and the pressure to succeed in a foreign environment. Meanwhile, Taichi remains in Japan, supporting Kohei from afar while pursuing his own aspirations. The distance between them highlights the importance of trust and communication, as they strive to maintain their connection despite the physical separation. The narrative explores themes of independence, personal growth, and the complexities of love, emphasizing the idea that happiness is not a fixed state but a continuous process of understanding and compromise.

Secondary characters, such as their friends and family, play supportive roles, offering perspectives on the challenges faced by Taichi and Kohei. These interactions add depth to the story, illustrating the broader impact of their relationship on those around them. The manga balances moments of emotional tension with heartfelt scenes, capturing the nuances of a mature romance built on mutual respect and effort.

"I Hear the Sunspot: Theory of Happiness" is a thoughtful continuation of Taichi and Kohei's story, focusing on their evolving relationship and the challenges they face as individuals and as a couple. The narrative emphasizes the importance of communication, resilience, and the pursuit of happiness, making it a compelling exploration of love and personal growth.