Castle

A spirit of a middle school dropout gangster who accidentally possesses an 18-year-old boy's body who killed himself, goes back to High school to take a revenge against the bullies with his gangster skills and encounters unexpected relationship.

Kim Duk-pal in Song Ee-hun's body learns that the boy has been bullied severely and decides to help him. However, his weak body isn't as capable as Duk-pal's, and Choi Se-kyung, a classmate is suspicious about his true identity.

Finally, Duk-pal succeeds in beating up Jae-min, the bully and his friends, but he refuses to repent. Meanwhile Lee Mi-gyeong, Chairman Bae's secretary hatesdespises Ee-hun and his mom and schemes to eliminate them.

Duk-pal narrowly saves Ee-hun's mom from being kidnapped and tries to find a way to deal with Mi-gyeong. Myeong-hyeon tries to control Se-kyung, and Jae-min, who was almost involved in Min-seo's kidnapping, anxiously loiters around Ee-hun's house.

Duk-pal notices Dong-su who once was his right-hand man has something to do with Mi-gyeong and orders to bring her to him. However, it makes Dong-su suspicious because he feels Duk-pal, the late from Ee-hun.

Duk-pal takes a trip with Se-kyung who ran away from his step-dad, and Se-kyung enjoys his newfound freedom with Duk-pal, doing things he never experienced before.

Duk-pal is accused by Byeong-cheol and his gang as Mi-gyeong schemed, and Duk-pal tries to meet chairman Bae Ee-hun's dad to figure this out.

Duk-pal gets cleared of accusation thanks to Jae-min‘s sincere repentance and testimony and also freed completely from Mi-gyeong‘s scheme. Just then, the spirit of the missing Ee-hun returns...