I never heard the term “through-line” before I became a professional screenwriter.

After I sold my first script, however, and started working with producers and studio execs on various projects, I started to hear it all the time.

You’ll hear this as well if you break into the business.

If you don’t already know, a through-line is essentially a theme or plot thread in a script that keeps reappearing.

In the first act you establish the characters and what their primary issue is: Do they dream of escaping their boring life and going off on some kind of adventure? Is there someone they’re in love with, but they don’t have the courage to make theirs?

The second act deals with their attempts to resolve this primary issue in their life. If it’s an action/adventure film, this results in a physical journey of some sort and an initiation into some kind of exotic or more exciting world than the one our protagonist is leaving behind. Whether it’s Luke Skywalker leaving Tatooine to join in the Galactic Civil War or Brian O’Conner hooking up with Dominic Toretto’s gang in The Fast and The Furious, this through-line usually involves a protagonist stepping out their comfort zone and learning the ways of this strange, new world.

The third act deals with the resolution of this journey. How has the character changed after it? Are they in a different place at the end of the film? A protagonist is usually facing their worst fears to get to this better place. If Martin Brody’s fear of the ocean is a through-line in Jaws, then it’s only natural he’d be on a sinking boat at the film’s climax, half submerged in water, with a great white shark heading straight for him. After defeating the shark, Brody jokes about his fear of the ocean while he swims back shore. He faced his fears and is now stronger for it.

These are some obvious through-lines in films with simple stories. But the same applies in almost every kind of narrative feature. Paul Thomas Anderson’s Magnolia establishes several characters all with different and unique issues and conflicts. So each character gets their own through-line, but the overall film has two prominent through-lines: Fate and reconciliation of the past. These themes are running parallel throughout. It’s the spine and soul of the film. In The Godfather Michael Corleone is established as a thoughtful young man who doesn’t want to follow in his father’s footsteps, and he will do anything from going to college to joining the army to avoid being part of the Corleone Crime Family. The through-line of the film is how despite his reluctance, Michael’s family loyalty and intelligence slowly but surely place him into his father’s role. At the end of the film, Michael becomes the Godfather. He’s the head of the crime family. There was no escaping his destiny.

Another classic film that has running themes of destiny and reconciliation of the past is The Empire Strikes Back.

The original Star Wars followed the more basic Tolkien Hero's Journey, which no doubt is why this is the framework that’s most duplicated in large, tentpole films. Short of its darker tone and cliffhanger ending, however, The Empire Strikes Back rarely gets replicated. It’s a more complicated, character-driven story. This has often been discussed, but there’s also a general consensus that Empire is not a self-contained narrative; it relies heavily on its predecessor and successor to tell the full story. As with any entry in an ongoing series, this might be true, but I’d argue that Empire has a more self-contained narrative than it’s usually given credit for. It’s also one of the best examples of through-lines running effectively throughout the film.

Here are the three main through-lines:

1.) Luke Skywalker’s quest to follow in his father’s footsteps and become a Jedi Knight.

2.) Darth Vader’s quest to find Luke Skywalker and turn him to the Dark Side.

3.) Han Solo and Princess Leia’s love story.

The film expertly introduces these three plot threads in the first five minutes. And all these plot threads come to a head in the third act and in Cloud City.

The opening crawl states that Darth Vader is obsessed with finding Luke Skywalker, and the first scene shows an Imperial Probe Droid landing on the ice planet Hoth (the droid being an extension of Vader’s search for Luke). We’re then immediately reintroduced to Luke and Han. They’re both on the ice planet, and Luke wants to investigate the “meteorite” (unknown to him, it’s the Imperial Probe Droid). So right out of the gate, we have Luke being drawn to his destiny, and what will result in the culmination of his quest to follow in his father’s footsteps (i.e. his confrontation with Darth Vader).

Right then and there, Luke and Han takes separate paths as do their through-lines.

Han returns to the Rebel’s hidden base. Once there, he comes in contact with Princess Leia, and their conflict is firmly established: Han wants to leave the Rebellion and deal with his debt to Jabba The Hutt. He can’t do this, however, without attempting to break through Leia’s emotional defenses and get her to admit she has feelings for him. Leia won’t relent. Angered, Han storms off. He knows he has to reconcile the sins of his past (being a smuggler, only thinking of himself), but his love for Leia keeps him from just leaving. Towards the end of the first act, when Han has a chance to escape as the Empire attacks the Rebel base, he’s compelled to go back and rescue Leia. This will ultimately result in his greatest victory and greatest failure. He will go on a journey with Leia, which will lead her to admit her love for him, but it will also leave him frozen in carbonite. Just think about it: Han’s story begins with him trying to flee a frozen world, but it’s his selfless love for another, that leads him to be incased in another kind of ice. Essentially, Han never escapes Hoth. And it’s the very past he was attempting to rectify (his smuggling, his debt to Jabba the Hutt) that comes back in the form of both Lando Calrissian and Boba Fett. Not only could he not escape an icy fate, Han couldn’t escape his past.

As it’s stated in Magnolia:

“We might be through with the past, but the past ain't through with us.”

Luke Skywalker, likewise, wants to right the wrongs of the past. In this case, he’s not dealing with his own sins. Unknown to him until the film’s climax, it’s the sins of his father. For the majority of the film, Luke believes his father was simply a good Jedi Knight who was murdered by Darth Vader. Luke wants to become a Jedi Knight, like his father, and defeat Vader.

At the start of the film, after Luke decides to investigate where the Probe Droid landed, he’s attacked by the Wampa. The next time we see Luke, in a bookend parallel to his friend Han, Luke is hanging in a frozen tomb of sorts. This is where we first see him use the Force and his father’s lightsaber. By doing so, he defeats the Wampa and escapes the icy tomb. This eventually leads to him seeing Ben Kenobi’s spirit, and his goal crystalizes: Luke is to go to Dagobah and be trained in the Jedi Arts by Yoda. As such, after the Battle of Hoth, Luke doesn’t follow the path of friends Han and Leia, rather he flies of with R2-D2 to Dagobah. On the swamp planet, he starts his training with Yoda, and appears on his way to becoming a Jedi Knight. But there’s another character’s through-line that interrupts Luke’s through-line: Darth Vader. In the metaphoric scene in the dark cave, Luke does battle with a vision of Vader, which is revealed to be himself? What can this mean exactly? How can he be like the villian who murdered his father?

Throughout the film, Vader is shown to be obsessed with finding Luke.

He will chases the Rebels across the universe to get to him.

He will kill his own officers whenever they fail him.

Luke’s metaphoric battle with Vader on Dagobah is in the center of the film.

Likewise is Vader’s discussion with the Emperor. It’s made clear in this scene that the Emperor has just recently become aware of Luke’s presence. As such, Vader’s quest to track down Luke is a personal one. Why is this?

Due to a trap that Vader sets for him in Cloud City, Luke doesn’t complete his training, and travels to Bespin in order to save his friends, who have been captured.

They’re simply the bait.

Vader’s only interested in one person:

Luke Skywalker.

Much like the Imperial Probe Droid that initially caught his interest, Luke is drawn towards Vader’s direction. Rather than follow Leia as she’s taken away by Stormtroopers, or follow the frozen Han as he’s taken away by Boba Fett, Luke chooses to follow some dark instinct that directs him to his destiny.

Luke and Vader have their epic light saber duel.

But this is not so much an action sequence as the film’s two main through-lines clashing until they become forever intertwined. Vader chops off Luke’s hand, which sends his father’s lightsaber down into a bottomless abyss. First is the metaphoric separation from a false past Luke believed in. Then is a literal separation as Vader reveals that he is in fact Luke’s father. Vader has just been toying with Luke. He wants his son to join him and help him overthrow the Emperor. Together they can rule the universe as father and son.

Everything Luke believed in is destroyed at that moment.

But does he relent?

No. Luke will not side with his father and turn to the Dark Side.

Instead he chooses what appears to be certain death, and allows himself to fall, not unlike his father’s lightsaber moments before, down into the bottomless abyss.

It’s only because of Leia (a future hope) that Luke survives.

An over simplification of the film is that the heroes lose at the end and the villains “win”. This is not true. Darth Vader is the film’s main antagonist, and he does not achieve his goal. In fact, he fails miserably. He isn’t able to seduce Luke to the Dark Side and the young Jedi-To-Be escapes to fight another day.

All the major characters in the film fail to a degree. Han and Luke, however, succeed in ways Vader didn’t manage to. Han finally got Leia to admit her love for him, and as such she is determined to save him. Luke has lost his hand and his physical battle with Vader, but he achieved an internal victory and did not succumb to the Dark Side. He no longer wants to follow in his father’s footsteps. He will not become the Michael Corleone of the Star Wars Saga.

Vader’s storyline ends with the Dark Lord in a thoughtful pose and for the first time sparing the life of one of his officers after failing to capture the Millennium Falcon.

The confrontation with his son has effected him deeply.

Vader began the film as a merciless villian in a mask.

He ends the film as a fallen hero in need of redemption.