Heroes and villains. Good versus evil. It’s a tale as old as time, and the foundation for the vast majority of comic book stories. The villain does something bad that threatens the utopian way of life citizens have come to know. Then, the hero steps in and saves the day by stopping the villain and restoring order in the chaos. It’s a formula that gets the job done. Cookie-cutter? Maybe. But cookies are delicious. We love cookies.

In real life, the dichotomy of good and evil isn’t as cut-and-dry, and every so often, the comics reflect this. Once in a while, these characters become more complicated than the Point A to Point B roller coaster ride you typically encounter in a superhero tale. And those are the storylines that endure, the ones that keep us guessing. They’re the stories that keep us asking questions and yearning for more answers and context, that leave us grasping at every rumor and hint until we know the whole story.

Since 1939, we’ve been captivated by Batman. The Caped Crusader is rich, powerful, handsome, genius, really rich, and, for all intents and purposes, good. He’s also the only superhero without any actual superpowers unless you count generational wealth. It makes him more human, and somehow, more relatable. He’s the only superhero that we could theoretically become. Well, not us, but Elon Musk or Prince Harry could if they got bored. Or Dwayne “The Rock” Johnson®.

This humanity makes Batman flawed like us, susceptible to being led astray by love, temptation, revenge, or jealousy. Batman is fueled by his trauma, his fire within stoked by violent memories and tremendous loss. His life of solitude and obsession is somewhat similar to the one led by his most relentless foe and, dare we say, longest friend?

Just one year after Bruce Wayne and his vigilante crime-fighting alter ego made their debut in Detective Comics #27, we were introduced to the Joker for the first time in Batman #1. They’ve been together since basically the beginning, and there’s no end in sight. We’ve seen countless iterations of Batman vs. Joker, from campy and funny to brutal and vicious, and still, we don’t know what’s really going on between them.

The pair can’t live with each other and they can’t live without each other. Batman and Joker are eerily similar yet polar opposites. The rivals are two sides of the same coin, and if we’re using Harvey Dent’s coin, they’re even more alike than we think.

Christopher Nolan took on the immense challenge of reimagining Batman and Joker in his own way for the Dark Knight trilogy and has admitted that casting both roles was extremely difficult. Many actors didn’t think they could compete with Jack Nicholson’s legendary portrayal of the psychopathic killer clown. And Nolan had trouble finding actors who could exude enough dark energy to outshadow the Batman costume as well. A few years before the late Heath Ledger accepted the challenge of bringing the darkest version of the Joker to life, he turned down Christopher Nolan’s invitation to play Batman in Batman Begins.

That fateful decision paved the way for Ledger to give one of the most impressive and impactful performances we’ve ever seen on film, one that earned him his lone Oscar and affected cinema forever. Superhero movies changed after Nolan’s Dark Knight trilogy. They became bigger and more immersive, darker and more violent. It sparked an arms race between DC and Marvel that is still being waged at the box office today, with every theater in America showing at least one blockbuster from both comic book empires.

The Dark Knight gave us our best view into the complicated relationship between Batman and Joker, the interrogation room scene still giving us chills and making our collective jaws drop over a decade later. The realization by both of them that they need each other is Earth-shattering, with Joker admitting he would never risk losing out on all this fun by killing the Bat, and Batman knowing he’ll never break his number-one rule by offing his nemesis once and for all (in the Nolan universe, anyway). Their symmetry in the film is beautiful, only emphasized by the half-and-half character arc of Harvey Dent, who spends almost exactly 50 percent of the movie fighting for each side.

Bruce is a billionaire, enjoying all of the extravagances the world has to offer while using his wealth to skirt the law and make his own rules. Joker is desperate and lost, without any of the options Batman has at his disposal. He uses his relatable struggles to amass a following of fellow misfits, whereas the Dark Knight works alone for the most part. Batman’s primary objective is maintaining order in Gotham City, and Joker’s is to create complete and utter chaos. Not motivated by money like the common criminal, Joker just wants to toy with people. He wants to know what you’ll do when you’re pushed to the edge, whether or not you’ll make the hard choice.

The Dark Knight ends with the Joker being captured, Two-Face dying, and Batman taking the brunt of the blame and retreating to the Batcave while Gotham “hunts” him. Sorry to ruin it for you if you haven’t had a chance to watch it yet. With Heath Ledger tragically passing before the film’s release, Nolan took the story in a new direction for the finale, incorporating Bane and Catwoman into the fold. But who knows what would have happened? Would Joker have escaped from Arkham Asylum? Spoiler alert: he always does.

It was recently revealed that there were discussions to build the DC Cinematic Universe out from Nolan’s Dark Knight universe, with Nolan and Zack Snyder contemplating the possibility ahead of production on Man of Steel. But they couldn’t have done it without the Joker–or with Joseph Gordon-Levitt as Batman, honestly. And it’s better this way, left alone like the masterpiece it is.

Though we’ll never know the true dynamics of Batman and Joker’s relationship or how they feel about each other deep down beneath the layers of lawlessness and morality, The Dark Knight gave us the most compelling glance yet into their psyches. The breaking of a person’s spirit, the lengths to which they’ll go when they have nothing left to lose, their grasp of good and evil crumbling under incredible pressure.

Nolan expertly kept us in the dark about the true source of Joker’s trauma, offering multiple misleading theories in The Dark Knight and never diving deeper. “The Joker we meet in The Dark Knight is fully formed…To me, the Joker is an absolute. There are no shades of gray to him — maybe shades of purple. He’s unbelievably dark. He bursts in just as he did in the comics,” Nolan said. “We never wanted to do an origin story for the Joker in this film”, because “the arc of the story is much more Harvey Dent’s; the Joker is presented as an absolute. It’s a very thrilling element in the film, and a very important element, but we wanted to deal with the rise of the Joker, not the origin of the Joker.”

Now, we enter new phases for both Batman and the Joker, though we’re moving forward while looking back. Todd Phillips’ standalone Joker film took a closer look at the villain’s origin story, exploring his trauma and dissecting the fibers of his fuse while it was being lit. The film, which will have a sequel, also hinted at the possibility Bruce Wayne and the Joker may even be related. We’ll see where that leads. Next year, Robert Pattinson will don the cape and cowl in Matt Reeves’ upcoming The Batman, set early on in Wayne’s crime-fighting days. The Joker isn’t supposed to make an appearance, but The Batman is the first installment in a rumored trilogy, so never say never.