The theory that a hero's mentor must die midway through a movie for the hero to be effective.

Examples:
Luke Skywalker and Obi-Wan Kenobi in Star Wars
Luke Skywalker and Yoda in Return of the Jedi
Obi-Wan Kenobi and Qui-Gon Jinn in The Phantom Menace
Rick Hunter and Roy Fokker in Robotech
Marty McFly and Doc Brown (sort of) in Back to the Future
You and Gorion in Baldur's Gate (CRPG)
Simba and Mufasa in The Lion King
Michael Corleone and Don Corleone in The Godfather

I know there are plenty more, but my brain is fried...