Following the death of their parents, Ryuuki lives with his older sister, Reiko. His life changes one summer when he becomes obsessed with a popular adult film actress named Kirill, who mysteriously appears before him in person. The story follows his sexual awakening and the eventual discovery of Kirill's true identity.

In Chapter 3 of "Shounen ga Otona ni Natta Natsu," the story continues to follow the protagonist and possibly other characters as they navigate their summer, a period marked by significant personal changes. Without specific details on the plot, the chapter likely delves deeper into the emotional and psychological transformations of the characters, possibly introducing new conflicts, relationships, or revelations that contribute to their growth.

While specific "choice guides" (as seen in visual novels) are limited for this title in standard search results, Chapter 3 typically serves as the "pivotal event" where the protagonist's transformation into an adult is triggered. The Love Triangle: