In 1930s Gyeongseong under Japanese colonial rule, Sehoon visits the imprisoned novelist Lee Yoon in Japan. A novel attributed to Hikaru—a supposedly deceased female writer and the lover of the late author Haejin—is about to be published, and Sehoon fears that the author’s identity will be exposed. Lee Yoon claims to possess Haejin’s final letter to Hikaru and demands the truth.

Years earlier, Sehoon had run away from home and found shelter with the League of Seven, a literary circle of young writers preparing a Korean-language magazine. Despite its name, the group had never gathered seven members. Sehoon became their assistant and admired Haejin, a talented writer suffering from tuberculosis.

Haejin drew inspiration from letters sent by a woman named Hikaru. In reality, Sehoon had created Hikaru and written the letters himself. What began as encouragement gradually became a separate voice within him—bolder and more demanding than Sehoon could be. Through Hikaru, he pressed Haejin to continue writing even as his illness worsened.

After the authorities investigated the group’s Korean-language writings, its members burned manuscripts and disbanded. Haejin nevertheless resumed writing after receiving another manuscript from Hikaru. Unable to watch him sacrifice his health, Sehoon finally confessed that Hikaru did not exist. Haejin implied that he had already understood the truth but wished Sehoon had preserved the illusion. Before dying, he wrote that the letters had given him love, language, and a reason to live.

Back in prison, Lee Yoon reveals that the supposed final letter was a trick intended to force Sehoon’s confession. He tells Sehoon that he is a genuine writer and must continue writing. Lee Yoon dies soon afterward.

Sehoon completes Haejin’s novel and is formally welcomed into the League of Seven. At the publication gathering, he delivers Haejin’s eulogy and accepts Hikaru as part of himself. In a symbolic reunion, Haejin, Hikaru, Lee Yoon, and the surviving writers finally complete the group’s long-imagined seven members.