"Invisible squirrels attack! Another big fight with Lord Garmadon, the ninja have come together to rebuild the Monastery of Spinjitzu and train with Master Wu. But when a training exercise gone wrong unleashes a group of angry Invisible Ninja Squirrels--that's right, angry Invisible Ninja Squirrels--the ninja will have to think fast or risk destroying the monastery all over again! It's an all-new, original, laugh-out-loud adventure!"--Page [4] of cover.