Makimura Koutarou begins summer vacation determined to confess his feelings to Serizawa Kaho, a classmate he admires yet finds intimidating. Lacking confidence, he enlists childhood friend Ai Senou to deliver his love letter. His life takes a drastic turn during an encounter with a mysterious girl named Ligene. Their collision covers him in multicolored powder and shatters her hourglass, unexpectedly propelling him forward to September 1st. There, he discovers Kaho became his girlfriend but tragically died in a car accident. Witnessing this future fuels his resolve to prevent her death.

Koutarou begins experiencing involuntary "day-dropping," skipping erratically between days of summer vacation. Guided by Ligene, who identifies herself as a Time Patroller, he learns to navigate these unpredictable time jumps. His mission evolves: altering events throughout the summer to ensure Kaho's survival while simultaneously pursuing a relationship with her.

Key relationships shape his journey. His longstanding friendship with Ai Senou strains under his singular focus on Kaho, causing her visible distress. His interactions with friend and boxing club member Takeshi Tokieda are also affected by his preoccupation with Kaho. The central conflict involves confronting Kaho's controlling father, whose influence contributes to the circumstances leading to her predicted accident.

Faced with the urgency to change the future, Koutarou's behavior shifts from hesitant to proactive and decisive. He actively intervenes in the timeline, demonstrating increased resolve and emotional investment in altering Kaho's fate. The compressed narrative centers primarily on his mission concerning Kaho, with other relationships receiving less exploration.