Season 1

Joon-hee is taken to the hospital after he gets beaten up. However, something other than his wounds is found. It turns out he is going through chronic renal failure just like his brother Joon-young. But this time, he has someone looking out for him. Despite Eun-hye's disagreement, Go Yoo firmly decides to donate his kidney to Joon-hee.

Go Yoo and So-yeon hang out more often and start to show their feelings for each other. Therefore, Joon-hee feels disappointed and left out. On Sunday night, Go Yoo and his friends are having a secret wine party at school, and he invites So-yeon to join them.

Subtle air flows between Yoo and Joon-hee due to their feelings for So-yeon. Meanwhile, Joon-hee becomes more confused about the changes happening to his body and mind. Later, Yoo discovers the secret So-yeon has been hiding.

Go Yoo takes a look at So-yeon's bag and finds Grandma Mo's necklace she lost. He immediately gets disappointed in her, but she does not give any explanation. Meanwhile, Joon-hee tries to think, talk, and act like Go Yoo.

Ever since the transplant, Joon-hee has been acting strangely. He has become more violent and started to act differently. Also, although he says he is looking out for So-yeon as a friend, he is being overprotective. Frustrated by Joon-hee's actions, Yoo aggressively tries to make him come back to his senses.

Yoo and Joon-hee have a huge fight in front of the whole school and end up in the teacher's office. Meanwhile, Joon-hee's grandmother collapses and gets an emergency operation.