From MangaHelpers:
Utsugi Souya has lived life avoiding the spotlight. He always tries to follow the rules and be a good samaritan, especially when it came to environmental issues. He would go as far as being late to school if it meant protecting the Earth by separating the trash in the correct order.
One day while walking home from school, he finds that someone has messed with his trash. His trash has been left behind because the different types of trash were mixed up. The perpetrator kept mixing up only Souya's trash everyday, so one day Souya decides to lie and wait to catch this annoying criminal. That day, he finds that the perpetrator was actually the recently transferred classmate, Tokisaka Koyomi, and she reveals that she has come from the future to stop all environmentally friendly activity. Now, Souya and Tokisaka will have to live together and destroy the environment so that Tokisaka can go back to the future.