Complex sentence: Tommy takes the opportunity to go into his bedroom and change his soiled pants when is ambushed by an alien--it (apparently) puts him in a trance, investigates his camera, and slips away, leaving Tommy with no memory of the encounter.
Split into simpler sentences: Tommy takes the opportunity to go into his bedroom and change his soiled pants. He is ambushed by an alien who (apparently) puts him in a trance, investigates his camera, and slips away, leaving Tommy with no memory of the encounter.