Adapted from Kabei Yukako's novel of the same name. After being guided by a mysterious message from the past on a cellphone's answering machine, Umi encounters Mahiro, a delinquent at her school. They gradually come closer, but more messages appearing on the answering machine will unveil past secrets...

Rino Sakuragi (Yuna Taira) is a high school student. She is good friends with Ren Kiritani (Alan Shirahama) who attends the same high school. They have been close since their middle school days. Rino holds special feelings for Ren Kiritani, but she is unable to tell him how she feels because he is popular. One day, Ren confesses to Rino that he likes her. They begin to date and enjoy their time together, but Rino is diagnosed with ALS (amyotrophic lateral sclerosis).

Atomu (Shuhei Nomura) was raised by his mother in a poor environment. He is now a young adult, but he stutters and has timid personality. To support himself, Atomu works part-time for a used junk collecting company. Atomu finds inspiration in rap music and his life changes.

Masaru and Yukiko have been married 44 years. They have three children who live independently from them. Masaru and Yukiko enjoy their peaceful days, but, one day, Yukiko tells her daughter Naoko that she wants to divorce Masaru.

Story continues with the students' own conflicts, Koro Sensei’s identity and the fate of the world. The time limit for assassination is approaching.

A mysterious creature destroys 70% of the moon. The creature then warns that if he is not destroyed by March of next year, Earth will be next. The creature also demands that he becomes the homeroom teacher for 3rd grade E class at Kunugigaoka Junior High School. The government is powerless to say no. The creature then becomes the homeroom teacher of 3rd grade E class which consists of failed students including Nagisa Shiota. The creature is called Teacher Koro. Meanwhile, the government requests that the students try to kill Teacher Koro even though he possesses super powers. The government offers a 10 billion yen reward for whomever successfully kills him. The students are confused by the situation, but decide to kill Teacher Koro. When their class begins with Teacher Koro, the students finds themselves having a good time with their new teacher.

High school student Shun Takahata is bored. Bored with the day-to-day monotony of school and life, he prays for change, for something exciting. Suddenly, he and his classmates are forced to play deadly children's games and facing terrifying creatures from a talking Daruma doll to a sharp-clawed lucky cat.