Complex sentence: Then Mitchell and Stanley find some terrific costumes at a local shop, and offer to help the proprietor organize a Halloween parade in return for letting them use the costumes for Halloween, unaware that they are about to get caught up in the weirdest adventure of all.
Split into simpler sentences: Then Mitchell and Stanley find some terrific costumes at a local shop, and offer to help the proprietor organize a Halloween parade in return for letting them use the costumes for Halloween. Little do they know that they're about to get caught up in the weirdest adventure of all.