An ordinary fifth-grader, Kim Si-hoo, discovers a suspicious ball of yarn stuck to his head one morning. As he struggles to remove it, a button falls from his clothes and touches the yarn ball, causing a surprising transformation... This magical yarn ball is named Mori! Mori, made of yarn, has the ability to transform into anything. Si-hoo's once calm daily life becomes unpredictable with Mori, the transforming yarn ball?! What if Si-hoo faces trouble? "Hehehe! It's my turn, Mori!"