Complex sentence: The story begins with a military truck transporting barrels of Trioxin, the soldiers occupying the vehicle who were smoking marijuana driving the vehicle through a downpour are unaware when a barrel breaks loose and falls into a river.
Split into simpler sentences: The story begins with a military truck transporting barrels of Trioxin. The soldiers (who were smoking marijuana) driving the vehicle through a downpour are unaware when a barrel breaks loose and falls into a river.