Fifteen-year-old Sei Saotome has a knack for failing at the worst possible moments. After a disastrous high school entrance exam, she ends up enrolling in a brutally demanding school where she is completely out of her depth. Struggling with the coursework and unable to connect with her classmates, she feels utterly isolated. Everything changes when she encounters two very different boys: Taiyou Udagawa, a sharp-tongued top student with a confident swagger, and Mizuki Ootaka, a quiet, astronomy-obsessed beautiful boy. When Mizuki notes that Sei's name sounds like the star Spica in the constellation Virgo, she instantly falls for him. Mizuki invites her to join the astronomy club, which currently consists of just him and Taiyou. Desperate for a place to belong, Sei accepts, and her previously bleak school life begins to brighten.

The story is set at a competitive high school, with the astronomy club serving as the central stage for the characters' relationships. The club's activities, from after-school observations to a summer training camp, provide the backdrop for their intertwined feelings. Sei's open crush on the gentle and dreamy Mizuki is the initial dynamic, but as she spends more time with the group, Taiyou, who initially seems annoyed by her, begins to develop his own feelings. A critical narrative arc introduces a past incident between Mizuki and Taiyou involving a girl, which made Mizuki wary of women. This history makes Taiyou hesitant to pursue Sei, as he had previously promised not to interfere with Mizuki's chances. The tension escalates when a senior named Subaru Misono makes a move on Sei during the summer camp, prompting Taiyou to fiercely declare that he will not back down this time. This pushes both boys to confess their feelings to Sei, leaving her caught between them.

As the series progresses, Sei must confront her own emotions and make a choice. The story follows the couple through the highs and lows of a first love, including the arrival of Taiyou's ex-girlfriend, Saki Amagawa, who creates further complications. The narrative arcs explore the challenges of maintaining a relationship amidst family moves, future ambitions, and personal growth. The manga concludes with the characters graduating and pursuing their respective dreams, with Sei aiming for medical university and Taiyou training as a doctor in America, ultimately building a future together. The complete story spans from their first meeting through the deepening of their bonds, portrayed against the backdrop of the night sky.