The Summer Hikaru Died

On a sweltering summer day, Yoshiki realizes that his best friend Hikaru—who had smiled beside him throughout their childhood—is no longer the same. Though this "Hikaru" looks and talks just like him, it's something else entirely. In their stifling, isolated village, Hikaru was the only person Yoshiki could ever truly trust. Desperate not to lose that bond, he chooses to accept the imposter—and vows to tell no one.

While discussing the strange and inexplicable death of an old woman in the village, a classmate named Maki confesses that he fears he might be cursed as well, and recounts the terrifying experience he had while taking an unfamiliar path home through the forest. In response to Maki's desperate plea, Yoshiki agrees to investigate. Together with classmates Asako and Yuki, who had been listening to the story, and Hikaru, who showed a keen interest, they venture into the forest...

As Yoshiki watches Hikaru happily eat a slice of watermelon, he's struck by a memory of the real Hikaru. A quiet guilt begins to gnaw at him as he's reminded that the real Hikaru is already gone–is it really alright for him to enjoy a "normal" life without him...? Unaware of Yoshiki's inner turmoil, Hikaru smiles at him and earnestly says, "Just being with you makes me happy." Overwhelmed, Yoshiki reaches out to Kurebayashi, a housewife he encountered at the supermarket.

Yoshiki enjoys a summer festival with his younger sister, Kaoru, who's excited to wear a yukata, and Hikaru. While eating shaved ice, Hikaru casually comments, "Just changing the color of the syrup makes it seem like it tastes different," which leads him to wonder, "If something looks the same, does it feel the same too?" He then asks Yoshiki how he figured out he wasn't the real Hikaru...

To help Yoshiki build resistance toward beings like himself, Hikaru once again urges him to reach "inside" his body. Although Yoshiki seemed more used to it than before, Hikaru's "interference" only escalates. Just as Hikaru had feared, Yoshiki has begun to attract impurities. Hikaru then heads to Yoshiki's house to exorcise the Wig Ghost that has appeared in his bathroom, but... 

With the choir competition approaching, Yoshiki, Maki, Asako, and Yuki take a break from practice by spending the night at Hikaru's house. Hikaru watches with a smile as fireworks light up the night sky, and the group enjoys a fun evening together. But then Asako—who had been quietly waiting for a chance to be alone with Hikaru—finally gets her chance to ask: "Just...who are you?"... 

Hikaru feels deeply guilty for having attacked Yoshiki. But, seemingly unfazed, Yoshiki simply says, "Let's skip school," and takes him into town for the day. While the choir competition unfolds back at school, the two spend the time watching a movie at the theater. Yoshiki can't help but feel that Hikaru's words and actions are "different" from humans...

Accepting Hikaru's wish to "stay here," Yoshiki decides to investigate what Hikaru really is. A memo left by the real Hikaru mentions "Nonuki-sama," but Hikaru's memories of it are vague. Then, Hikaru's grandfather suddenly asks him, "Where did you put Hichi-san?"—deepening the mystery even further. Meanwhile, signs of impurities begin to gradually appear at school... 

Yoshiki introduces Kurebayashi to Hikaru. If the impurities continue to spread, many more people could die... Kurebayashi, who once experienced the heartbreak of her late husband "coming back" as an impurity, asks them what they want to do. After hearing their answer, she agrees to help them. Meanwhile, Tanaka—after a chance encounter with Yoshiki that gives him a lead on the entity he's been searching for—begins to make his move...

After sleeping for two full days, Hikaru finally wakes up—much to Yoshiki's relief. But Hikaru is clearly weaker than before and now feels pain. As Yoshiki and the others reel from a sudden attack by a mysterious man wielding a seemingly cursed sword, Tanaka confronts Takeda. It seems Tanaka has his own objective—one that goes beyond the mission assigned to him by his company...

Though hesitant, Yoshiki asks his father about the Indo family's murky past and secret ritual. Later, Hikaru loses control at school and feels deeply shaken.

While everyone else attends the school assembly, Asako finds Hikaru wandering the halls. Alone at the beach, Hikaru reveals his final plans to Yoshiki.