Tokisaka-san wa Boku to Chikyuu ni Kibishisugiru.

Utsugi Souya has always preferred to stay out of the spotlight, quietly dedicating himself to doing the right thing. A steadfast rule-follower and devoted environmentalist, he takes his ecological responsibilities so seriously that he is willing to arrive late to school just to ensure every piece of garbage is sorted into the correct category. His peaceful routine is shattered when he discovers that someone has been deliberately sabotaging his efforts. Each day, his carefully separated waste is left behind at the collection point, maliciously mixed together by an unknown culprit targeting him alone. Determined to put a stop to this daily frustration, Souya decides to lie in wait and catch the offender in the act. To his astonishment, the vandal turns out to be Tokisaka Koyomi, a recently transferred classmate with an unbelievable secret: she is a time traveler from the future. However, her mission is far from noble. She has journeyed back in time specifically to halt all environmentally friendly activities. Now thrust into an absurd partnership, Souya finds his lifelong principles turned upside down. In order for Tokisaka to successfully return to her own era, the two must live together and work toward a goal that defies everything Souya stands for—actively destroying the environment.