Before the main characters were born, a devastating pandemic swept across the globe, killing a large portion of the population and fundamentally reshaping society. The story takes place in a near-future Japan where the virus remains an ever-present threat. To prevent infection, wearing a mask in public has become an absolute and unquestioned social norm. This practice has evolved into an unbreakable taboo, where showing one's mouth to another person is considered an act of extreme intimacy, treated with the same gravity as revealing one's genitals. For the generation born after the pandemic, seeing the full face of anyone outside their immediate family is an exceedingly rare and deeply private event.

The central conflict revolves around two high school students, Haruto Hata and Emi Natsuki, who accidentally share this forbidden intimacy. Hata, a member of his school's drone racing club, catches a glimpse of Natsuki's face as she drinks from a water fountain. Natsuki, a track team member who secretly yearns for the lost world of the pre-pandemic era she has only seen in old movies and stories, is horrified but also intensely curious. Instead of simply ignoring the incident, she seeks out Hata and asks to see his face in return. This mutual secret becomes the foundation of their clandestine relationship, as they begin to meet in private to experience the simple, once-ordinary act of seeing each other without masks. They go on picnics and share other small pieces of the old world, all while navigating the intense social danger of being discovered.

The main characters include the male lead, Haruto Hata, a somewhat reserved boy who develops strong feelings for Natsuki. The female lead is Emi Natsuki, a girl fascinated by pre-pandemic culture and increasingly drawn to Hata as they share their secret. Their relationship is complicated by the arrival of Erika Kitazawa (or Erika Hinata in some sources), an underclassman in Hata's drone club. After Hata accidentally sees Erika without her mask, she blackmails him into a date to cover up her own mistake. This event leads to a romantic rivalry with Natsuki, as Erika also develops feelings for Hata. Another key figure is Airi Mochizuki, a classmate of Natsuki who introduces her to an underground anti-mask protest group called TEETH, which advocates for the right to go unmasked, bringing a dangerous external pressure into their lives.

The narrative unfolds in several distinct arcs. The initial arc establishes the bizarre and oppressive world, detailing the chance encounter between Hata and Natsuki and the beginning of their secret, mask-free relationship. A subsequent arc introduces Erika as a rival, leading to a love triangle filled with jealousy and competition over Hata. The story then escalates beyond a simple romance when the characters encounter the anti-mask group TEETH. This conflict becomes dangerous, as Hata is infected with a trace amount of the V-21 virus during a protest, leading to his quarantine and isolation in a medical facility. A tragic subplot occurs within the quarantine arc, where Hata befriends a fellow patient, a dancer named Shina Nanami, who ultimately succumbs to the virus after revealing her face to him one last time, deeply affecting him.

Later volumes see the story take on a more adventurous and somber tone. The characters venture into abandoned, disinfected zones of the city, encountering the lingering physical and psychological dangers of the pandemic. Volume 7's synopsis describes the group returning from such an expedition, forever changed by the losses and trauma they experienced. The series continues to follow Hata and Natsuki as they struggle to communicate their feelings, and new characters like Iura are introduced to further challenge their relationship in a society still grappling with the new normal.