Sekai ka Kanojo ka Erabenai

One day, Kouki Nakagawa was determined to declare his love to Ayumi Fujisaki, his childhood friend. Suddenly, a mysterious girl appears before them and mentions that she is Kouki's girlfriend, leaving both perplexed. After trying to talk to Ayumi again, Kouki confesses his love to her; But Jindou, the mysterious girl, appears to warn Kouki that if he does not accept to be her boyfriend and does not forget Ayumi... the world will be destroyed!