Complex sentence: Timmy's imaginary friend, Gary, is first set loose in the episode, "Imaginary Gary" the reason why he's so vengeful is because Timmy abandoned and imprisoned him in his head, where Gary sets out to ruin Timmy's life.
Split into simpler sentences: Timmy's imaginary friend, Gary, is first set loose in the episode, "Imaginary Gary". The reason why he's so vengeful is because Timmy abandoned and imprisoned him in his head, where Gary sets out to ruin Timmy's life.