"Only the Ring Finger Knows" is a manga series that explores themes of love, identity, and connection through the lens of a romantic relationship between two high school boys. The story centers around Yuichi Kazuki, a popular and outgoing student, and Wataru Fujii, a reserved and introverted classmate. The two initially appear to have little in common, but their lives become intertwined when they discover they are wearing identical rings on their left ring fingers. These rings, unbeknownst to them, are part of a pair known as the "Promise Rings," symbolizing a deep bond between two people.

The narrative unfolds as Yuichi and Wataru gradually grow closer, their relationship evolving from curiosity and confusion to genuine affection. Yuichi, who is initially dismissive of the idea of a fated connection, begins to question his feelings as he spends more time with Wataru. Wataru, on the other hand, struggles with his own insecurities and the fear of rejection, particularly given Yuichi's popularity and seemingly carefree demeanor. The rings serve as a metaphor for their emotional journey, representing both the challenges and the strength of their bond.

As the story progresses, external factors and misunderstandings test their relationship. Miscommunications and societal pressures create tension, but the two ultimately learn to navigate these obstacles together. The manga delves into their individual struggles, such as Yuichi's fear of vulnerability and Wataru's difficulty in expressing his emotions. Through their interactions, both characters experience personal growth, learning to trust and rely on each other.

The supporting cast includes classmates and friends who provide additional context to the main characters' lives, highlighting the contrast between their public personas and private struggles. The manga balances moments of lightheartedness with deeper emotional exploration, creating a nuanced portrayal of young love. The resolution of the story emphasizes the importance of communication and mutual understanding, as Yuichi and Wataru come to terms with their feelings and the significance of the rings they share.