The Sisters Grimm are two sisters who move from foster family to foster family. Each family they meet ends up being crazier than the next. One family in particular had a dad who was a amateur knife thrower. The sisters were his new targets before they ran away. When their real grandma finally finds them and brings them to her home they discover a town where fairy tales are reality (hence the sisters "Grimm"). Imagine if your lawyer was Robin Hood and you were friends with the Big Bad Wolf and Little Red Riding Hood. Imagine finding out that your Grandma was a "fairy tale detective" constantly working to keep the "Everafters" safe from the world around them and the world around them safe from the "Everafters." It's a delicate balance.
Michael Buckley intended to make an action pack mystery/comedy and definitely pulled it off. If you liked Harry Potter then you will love reading The Sisters Grimm.