NOTE: The 2019 Joker movie is cannon, but I've changed the ending to look like this: After the scene with Joker on the police car, comes the credits, and a post-credit scene where Joker thinks it's time to, "Change this costume a little bit." And we get the iconic Joker outfit. In the Superman story, Kal-El lands on Earth 13 years prior to Joker.
Now, fast forward 18 months after Superman, and thanks to his partnership with Wayne Enterprises, Superman has evolved from being the local Superhero of Metropolis, to a global defender of the innocent. The Batcave was originally designed to be a base camp for Superman.
Bruce Wayne is Superman's closes connection with Wayne Enterprises, but when they're not being business partners, they are good friends, and would do things such as eat out together.
There is a party scene at Wayne Manor, where Superman is being handed an award for heroism, which Bruce is there to present to him. Superman gives a speech where he talks about how he never knew Krypton, and figured that he wouldn't like it if he did, considering that Zod was in a station of power. He then tells the audience how proud he is to continue defending Earth, and doing so closely to Bruce and his company. Superman wraps up his speech holding up a glass of champagne and saying, "A toast, to a very, very, bright future!" And the audience cheers.
The camera then zooms out, showing this speech from the perspective of a small tv. We then hear Joker's voice, talking about how he sees Superman as a threat to his cause, and then he finishes the scene saying, "Well, pack up, boys! We're going to Metropolis." And then a bullet is shot into the TV where Superman is, and Joker laughs before the screen cuts out.
The next scene is back at the Batcave, where Superman walks in on Bruce fighting a robot, and he wins by tripping it onto the floor. When he stands upright, Superman walks clapping slowly, and says, "Nice Job, Bruce!"
BRUCE: Thanks, it something I've been doing since, my parents died. It's what I do as a way of calming myself.
SUPERMAN: Oh. Yeah, you've told me that story. But ya know what?! I think you can take three of those things!
BRUCE: Huh?! Woah! I've never done that before!
SUPERMAN: (Puts his hand on Bruce's shoulder) Hey Bruce, here's my advice on that: If you want to become better at something, you need to push your physical limit. Come on! Give it a try.
BRUCE: (Pauses, and takes a deep breath) Okay, I'll try.
Superman then steps out of the way, and watches Bruce activate three robots, and easily take them out one by one. Bruce is stunned by his performance, and thanks Superman for the advice.
Suddenly, an alarm goes off, and Bruce says that it's the alarm that indicates something bad going on out there. They run to a big computer, and see live footage of a massive, chaotic riot in Metropolis. During which, a man in clown makeup, and green and purple clothing, stands on top of a truck, and uses a bullhorn to give a speech.
The man introduces himself as 'The Joker', and says that he started this movement back in the early 80's, and why he did so. Then, Joker finishes with, "Superman! If you're listening, heed my warning: If you dare to try and stop us, you have officially become the bad guy! Thus, we'd have no choice but to bring you down!"
Back at the Batcave, Superman hears Joker's entire speech, and chooses to go stop him anyway, thinking he can reason with the insane clown anarchist.
Superman returns to Metropolis, and fights through the rioters trying to kill him, as he searches for the Joker. Without warning, Joker shoots Superman in the back with a giant gun he immediately brags about stealing from Lex Corp, as Superman's vision faded to black.
Superman wakes up back in the Batcave, on a hospital bed-like machine, with Bruce sitting next to him. Bruce explains that he was knocked-out for a week, and when he saw what happened to Superman on the computer, he called Sargent Stone and told him and his boys to go fetch him.
BRUCE: I wouldn't try to move if I were you, that gun was loaded with Gamma Radiation, and has injured you very much.
SUPERMAN: What about Joker?
BRUCE: He's disappeared, but people have reported seeing him or some of his followers her in Gotham. (leans closer to Superman) Clark, I think this Joker, is the same one that got my parents killed, and with your permission, I wanna get justice for them, and you.
SUPERMAN: If you do, please make me a promise, Bruce.
BRUCE: Yes?
SUEPRMAN: Please... don't kill him.
BRUCE: What? Why?
SUPERMAN: Because if you do, you'll just become no better than Zod; someone with good goals, but evil ways of achieving them. If you ever find him, I ask you to achieve justice, not retribution.
BRUCE: I don't know, I'm not sure if I can keep this promise.
SUPERMAN: Then remember my advice from earlier: If you want to become better at something, you need to push your physical limit.
Bruce, pauses to think, before nodding his head and saying, "Okay, I promise."
Cut to Bruce sitting at the computer, and his butler, Alfred, comes up to him from behind, and asks, "So Master Bruce, how do you plan to capture Joker?
BRUCE: Well Alfred, Superman was a superhero. I was thinking that maybe, I should be one too.
ALFRED: You wish to become a superhero, Master Bruce?
BRUCE: In tribute to the best one ever.
ALFRED: But, what will your persona be?
Bruce was about to say, "I don't know." But was cut short when bats came flying through the cave (something they find annoying, which is set up earlier, but I haven't figured out how yet), and Alfred complains, while Bruce sees the silhouette of a Bat showing its wingspan, and looking down, and Bruce envisions the Batman logo for the first time.
Then after seeing Bruce smiling and saying, "Yeah." We then see a montage of Bruce making the first Batman armor, from the Batarangs, to the grapples, to the suit itself.
The montage ends with Bruce looking into the camera, which is on the nose of the helmet, and Alfred walking up behind him, looking at the suit, and saying, "Impressive, Master Bruce."
BRUCE: Thanks, I even have a name for him already.
ALFRED: I'm pretty sure you do, but the computer has just detected some of Joker's thugs just outside the Gotham hotel, talking about meeting him somewhere at midnight.
BRUCE: Than what are we waiting for?! We have no time to waste!
Bruce than grabs the camera, before the screed fades to black, and then we see a shot of the camera panning around the corner, showing the Joker thugs standing under a streetlight that's on, because it's at night.
One of the thugs says, "Alright, let's get outta here. We need to rendezvous with the boss and the others soon." Before a shadow quickly passes over them, which started them into a fighting position.
They then start to hear sounds coming from all directions that make them slowly go from startled, to straight-up terrified. Then, out of nowhere, a man in a black cloak and solver armor appears and punches one of them straight to the ground, and we continually cut from the thug trying to get up, and the others getting beaten up by this man, without showing his face.
When he fully got up, the others were on the ground. The thug then tries to hit the figure in the back of the head with a bat, but he blocks it, and holds the thug against a wall with his left hand.
The thug, while being choked, then says, "Wh-W-Who the hell are you?"
The camera than turns to show the figure's face, and we see Batman's head, torso, and arms all in frame with the criminal. Then through a voice modulator that makes his voice sound deep and gritty, he says, "I'm Batman. Now, where do you plan to "rendezvous" with Joker?!"
THUG: (grunts) Grill me all ya want, Bat-freak! I'm not telling you anything.
BATMAN: Oh really?
Batman then takes one of his batarangs, and shows that he has a gadget that can spin it's dangerous blades around, as he brings it closer to the Thugs face.
THUG: (gets scared on the cheek by the blades, and scream loudly because of how violently it spins) Okay! Okay! I'll talk!
BATMAN: (Takes the blades back) Well then, spill!
THUG: Joker is wanting us to meet with him and the others at a facility for a company called 'Ace Chemicals'. He says he's looking for a chemical called 'Venom'.
BATMAN: What does he want with it?!
THUG: I don't know! I've told you all I know!
BATMAN: Then you've told me all I need from you.
Batman gives the thug a knock-out punch, before putting two fingers on the side of his helmet, and saying into a communicator, "Alfred, I've just been informed that Joker is after a chemical known as 'Venom', what can you tell me about it?"
Then, we cut back to the Batcave, with Alfred at the computer, searching for history on Venom. He then goes on to say, "Well Master Bruce, it was originally brewed in the Caribbean Island nation of Santa Prisca, with the intention of creating a super soldier. For the test, it was used on a prisoner of Peña Duro.
The computer than shows footage of the prisoner, becoming a big, hulking monster, and killing so many people, before throwing something into the camera. All this leaves Alfred speechless, as Batman asks, "Alfred? Alfred, are you there?!"
Alfred responds with, "Sir, y-you might want to see this." Before sending the footage to Batman, who sees it in a screen in his helmet.
BATMAN: Oh, that's not good.
ALFRED: What do you think Joker plans to do with this?
BATMAN: Well, he already has an army, if I had to assume, he wants to use it to create a new army of a thousand of those things.
ALFRED: Why would he do that, master? He has no way of controlling them.
BATMAN: (Walks backward a few steps from the wall) We're dealing with an insane clown Alfred, how could he ever follow reason.
ALFRED: Agreed. We have 3 minutes until midnight sir, we must stop him.
BATMAN: (takes out his grappling hook) Oh, (points to upward) I will.
Batman than shoots his grappling hook to the top of the building, and it pulls him up to the top of the building, before shooting him into the sky, and while he is silhouetted by the moon, uses his cape to glide.
Later, he comes to the Ace Chemicals facility to see that Joker's goons have arrived there already arrived, and were taking containers of Venom, to a truck outside the front doors. Inside, he finds half of them taking the containers, and the rest standing guard, but could only hear Joker giving orders through walkie-talkies.
One by one, Batman silently takes down the guards, looking for where Joker could be, until one of them come to Joker's location in the area.
THUG: Boss! There's some psycho in a black cloak, taking us down one at a time!
JOKER: (Turns to face the thug) Wait. There's someone here, taking us out?
THUG: Yes! He's freaky, too!
JOKER: (puts on an angry face) I should have known Superman would have someone hiding in his back pocket, just ready to do his dirty work! (shrugs while turning away) Oh well, guess I'll have to go with plan B.
THUG: Plan B? You didn't say there was a plan B.
JOKER: Exactly! I don't want any witnesses.
THUG: Wha-?
But before the thug could finish, Joker turns and shoots him with a revolver, and says, "I said I don't want any witnesses! Get it?"
Meanwhile, Batman hears the gunshot as it happens, turns to a small control booth on the other side of the facility, and uses his grappling hook to swing there spider-man style. But when he sees Joker with his back turned to him, he turns him over to find a wax dummy he used as a decoy. Suddenly, he hears the truck from before starting, and he swings back there to find Joker driving off, already at full speed. Batman runs after the truck, while grappling to its back, and basically skiing from the back, while his boots leave lines in the street.
While driving, Joker sees Batman in the rear-view mirror, and after groaning says, "I don't know who this guy is, but doesn't he ever quit?!"
After pulling himself closer to the truck, Batman jumps up, retracts the grapple, and uses another to swing around to pull Joker out the other end, by the neck. Once they're both out of the truck, it spins out of control and while it does so, we see the containers of venom swerving around inside, before cutting back out, to see the truck about to fall over a ledge, into a lake, before Batman uses a grapple to stop it.
After this action sequence, Batman uses his grapple to get closer to the truck, then he holds Joker up by the neck, and says, "Hey! Heard of me? You can call me... Batman!" Before angerly slamming Joker against the truck, and raising a fist to deliver the killing blow. He has a killer look on his face, Joker is terrified and has his hands up, and it looks like Batman is finally going to get his long-awaited revenge. But then, he flashes back to the promise he made Superman, and now spends some time choosing if he should end it right now, or keep his promise. In the end, he just knocked Joker out, and called the police, before disappearing, and watching the police taking him away, returning the Venom, and talking about how he could have ended up there with the truck.
Later, we see Joker in an interrogation room, wearing a strait jacket that has the Arkham logo on it, and female psychologist walked into the room, introduced herself as 'Harleen Quinn', and interviewed him. We then see a montage taking course over a few months, and we see Joker come on to Harleen (who he calls 'Harley'), then he convinces her to help him get out, and we see her transformation on screen.
After Bruce Wayne hears that Joker has escaped, he decides that because he only became Batman to stop Joker, maybe he should be this alter ego of his for just a little while longer. After which, he goes on a big hunt for Joker, as he goes around America, performing more crimes alongside Harley. This goes on until Harley tries to get all the credit, and Joker gets his boys to kick her out.
Everything ends where it also began: Lex Corp, in Metropolis. At Lex Corp, Joker find toxins he want to use to get rid of Batman, before heading back to Ace for the Venom. The last fight takes place in a giant room, on a catwalk, over a giant pool of toxins, where Batman accidently hit Joker so hard, he fell into the Toxins, and even though Batman tried to save him, he couldn't. After this, Joker's army separates into smaller gangs.
Later, back at the Batcave, Bruce learns that Superman has finally recovered, and they meet each other.
BRUCE: You're not gonna believe the adventure I just had!
SUPERMAN: Don't worry, I heard every word.
BRUCE: You did?
SUPERMAN: Apparently, I have super-hearing. I learn something new every day.
BRUCE: Hey, um, sorry I wasn't able to keep my promise.
SUPERMAN: Don't worry! You tried to save him, that shows me that you care about justice. I applaud you for that.
BRUCE: Thanks, man.
SUPERMAN: So, it's Batman, huh?
BRUICE: Yep.
SUPERMAN: Well, I could always use help in protecting the world. Do you think you could, maybe, continue being Batman for a little longer?
BRUCE: (pauses to think) Yeah, I think I could.
The final scene on the roof of a building, with the sun rising in the distance. Then, Superman and Batman, in costume, walk into frame from behind the camera. The next shot shows the front of them, and they pose majestically: Superman puts his fists on his hips, and Batman grabs his cape, then does a T-pose with it. The final shot shows them both being silhouetted by the sun, and when it cuts to black, the Batman logo blinks on screen.