Complex sentence: During a summer the boys are staying with Grandpa Mori, the boys encounter a group assaulting a girl named Jo at a pizza parlour, after fending off the men they are praised for their martial arts techniques, which gives them big heads.
Split into simpler sentences: During a summer the boys are staying with Grandpa Mori when they encounter a group assaulting a girl named Jo at a pizza parlour. After fending off the men, they are praised for their martial arts techniques which gives them big heads.