Most stories, be it on the screen, stage or page, have plots that can be broken down to five major turning points. These turning points alter the protagonists' direction, either by changing his goal or introducing new obstacles that set him further away from achieving it.
The Inciting Incident - This is the event that kicks everything off. It will typically turn the main protagonist's ordinary world upside down and open a door to a new realm of the unknown. It occurs about half way through the first act and sets up the conflict to come.
In Back to the Future, it's the moment when Marty witnesses Doc invent time travel.
In The Godfather it's when, while at his sister's wedding, Michael Corleone tells his girlfriend that Luca does the dirty work for his father saying, "That's my family Katie, it's not me." Of course, the story is Michael's descent into that world.
The Lock In - Occurring at the end of Act 1 and propelling the story into Act 2 the lock-in outlines the main goal for the protagonist. The door of the inciting incident that opened and invited the protagonist into a new realm - that closes, locking the main character in the new realm, and if it he wants to return home, he'll have to achieve his aim.
In Back to the Future it's when Marty travels back in time to prevent Doc's murder at the hands of the Libyan terrorists from whom Doc stole plutonium for the DeLorean. This leads him to stealing the affection of his teenage mother Lorraine, when it was supposed to fall on his teenage father George.
In The Godfather it's when Michael's father - Don Vito Corleone - is gunned down in the street. When Michael hears of this he rushes to his father's bedside and into the family life he'd tried to distance himself from.
Midpoint - Also known as the first culmination, the midpoint occurs in the middle of Act 2 and marks either the first major success, or the first major failure of the protagonist so they're either closer to or further away from their goal.
In Back to the Future it's when Marty realises his brother is gradually fading from a photograph he has, because Marty's disrupted the space/time continuum by meeting his mother and father in the past.
In The Godfather it's when Michael murders The Turk and dirty cop McCluskey and flees to Italy where the family will protect him.
All Is Lost - This is the point that propels us into the third and final act, when the obstacles against our protagonist seem insurmountable, they're staring defeat in the face, victory couldn't be further away and you're wondering "How the hell are they gonna get out of this?"
In Back to the Future it's when Biff has his goons lock Marty in the trunk of his car allowing Biff to make unwanted advances on Lorraine, and with Marty's dad free to play the hero, however it's already been well established George doesn't haven't have the balls to stand up to Biff.
In The Godfather it's when Michael travels to Las Vegas to buy out Moe Greene's stake in the Corleone's casinos and Greene derides the family as a fading power. To add injury to insult, Michael sees his brother Fredo falling under Greene's influence.
Climax and Resolution - In the most exciting way possible the climax resolves the ultimate problem posed by the inciting incident, as well as wrapping up any loose story threads set up along the way. Our protagonist either leaves the new realm to return home a changed character, or stays and becomes King. So the inciting incident provokes the question - what will happen? and the climax declares - this!
In Back to the Future a storm brews and a fallen branch disconnects the cable Doc has run from the clock tower to the street in order to harness a lightening strike which Marty needs to pass under in the DeLorean in order to get back to the future. But Marty can't start the car and Doc struggles on the clock tower's ledge trying to reconnect the cable. Marty manages to get the DeLorean up to 88mph as the lightning strikes the clock tower and electricity flows through the cable. Doc's murder is prevented thanks to Marty's letter telling him to wear a bullet proof vest. Marty wakes up the next morning and finds his family changed - George is a successful author full of self-confidence, Lorraine is happily in love, and Biff, rather than being a bullying superior, is now subordinate to George and Marty.
In The Godfather Michael orders the assassination of the New York dons, Moe Greene and Tessio. Michael questions Carlo on his involvement in Sonny's murder and after admitting he was contacted, Peter Clemenza kills him with a wire garrote. Kay then confronts Michael, who denies killing Carlo, an answer she accepts. As Kay watches Michael warily as he receives his capos, who address him as the new Don Corleone. All this occurs at the Christening of Connie's son, for whom Michael is the godfather.
