Kamejiro, who lives with his girlfriend Kokoro, finds out that she has been invited to a high school reunion. She has just turned 20 and is not used to drinking alcohol yet, so Kamejiro is worried about letting her go to a drinking party. He tries to contact her many times during the reunion but is unable to get through... The next day, he confronts Kokoro when she comes home in the morning, but she doesn't say anything. Kamejiro becomes even more worried, and then he witnesses her bringing home a man (her ex-boyfriend) whom she met at the reunion, and he gets depressed after seeing the whole affair.