Complex sentence: Kevin then spies on Marv and Vera in his bedroom and hears them planning about kidnapping the royals; unfortunately for them, Kevin makes sure that their plan was a failure and has them fall out and break the window.
Split into simpler sentences: Kevin then spies on Marv and Vera in his bedroom and hears them planning about kidnapping the royals. Unfortunately for them, Kevin makes sure that their plan was a failure and has them fall out and break the window.