-Outside-
Hulk and Batman walk out of the courthouse and head towards the Gotham Cathedral, with Hulk leaping across buildings and Batman using his athletic skills as well as his Grapnel Gun to scale buildings before making it to the front entrance of the Cathedral, overhearing some thugs bring up Harley Quinn. After the two easily dispatch the goons, Batman contacts Alfred.
"We're at the church. It looks like Harley Quinn is inside." Batman says into the communication device in his cowl.
"That dreadful woman is no doubt setting a trap for the two of you." Alfred says warily.
"Don't worry, Alfred. Quinn never was too smart. We'll be okay." Batman says as he ends the call.
"Just like old times..." Hulk says with a sigh.
"Quinn will underestimate us, Hulk. Remember, no killing." Batman says sternly.
"Heard you the first time, Bat. At best, she'll get a bruise." Hulk says as he and Batman walk into the Cathedral. Once inside, Harley Quinn backflips over some of her henchmen, charging towards the two heroes.
"Coming through!" Harley shouts as she runs at the two, priming up a kick, but Hulk catches her leg and lightly tosses her to the floor. "Ow!" She groans.
"Deja vu, eh, Quinn?" Hulk remarks, remembering what happened the last time she tried the same exact move on them a year prior at the asylum.
"Let the lady go, you freak! Or these people all get a bullet in their heads!" One of the henchmen says, pointing to various medical officials that they've taken captive.
"I think you should do what he says." Harley says as she picks herself up off the floor and dusts herself off. "It'd be a shame to get blood all over my nice new outfit. What do you think, Bat-Brain and Musclehead? Like it?" Harley asks, wanting the heroes' opinion on her costume.
"Yep." Hulk says with a smile.
"What am I saying? Of course you do. The big man does. Who wouldn't?" Harley asks in a giddily way. "So anyway, here's the deal. Mister J is really not up to a visit right now. He's not feeling himself...Well, actually, he was earlier, but that's not what I meant." Harley says, to which Hulk makes a slight grimace of disgust. "He's not doing so good and that idiot Doctor I sent from here didn't help. I've seen more smarts from these bozos." Harley says, pointing at her henchmen. "I've gotta run, boys. If they try anything funny, kill them!" She says as she heads for the front door, blowing a kiss at the two heroes before departing.
Once Harley leaves, the goons keep their guns trained on the heroes. "Don't you move. Just stay where you are...and this can all end quickly." One of the goons says shakily.
Hulk grins as he prepares to attack, only for Batman to stop him. He uses his detective vision, noticing several hostages being held at gunpoint; they will die if Hulk attacks. "Hulk, no. They have hostages, remember? A forward assault will get them killed. We need to be smart about this."
"What do you suggest, a game of hide-and-seek?" Hulk asks, which gives Batman an idea.
"Hold on." Batman says as he tosses a smoke pellet on the ground, temporarily obscuring the thugs vision while Hulk and Batman escape to several reinforced gargoyle statues above them. "Good thing these gargoyles can hold Hulk's weight." Batman says to himself. "They don't know where we are. Good. Let's keep it that way. Time to survey the room, plan out our tactics." Batman activates his detective vision again to begin a strategy. "Hmm...four thugs, all armed, two hostages. This is gonna be easy." He says into his earpiece, which Hulk hears.
"What's the plan, Bat?" Hulk asks as Batman looks at one of the hostages, who is being held at gunpoint behind a confessional booth.
"There. That idiot thinks he's safe behind the confessional. He's just making it easier for one of us to get in behind him. That's the one you'll take down."
Hulk nods with a light smile. "What about you?"
Batman looks at the other hostage taker, who is standing under a wooden scaffolding. "There. I can glide to the scaffolding above him without being seen and take him down from there."
"Let's save some hostages, then." Hulk says as Batman glides to the scaffolding while Hulk gets behind the confessional booth. The two heroes than take down the hostage takers simultaneously, leaving the other two thugs alone as they nervously look around, fearing for their life as Hulk jumps upward and presses his palms to the goons' faces as they fall backwards, knocking them out cold. "Rrrr...I hate holding back." He says, grumbling at the idea of holding back his unparalleled strength when dealing with ordinary criminals.
"It's more preferable than having their blood on your hands, Hulk. Trust me on that." Batman says before turning his attention on the former hostages. "The room is secure, you're safe now!" He says as the two heroes untie the hostages before rushing into the tower.
After using their athletic skills, the two heroes reach the top of the tower, which has two mannequins dressed like Joker with TV's for heads, several blocks of TNT, and a sniper rifle.
"There's the gun. Looks like it was being remotely controlled by Joker." Batman says as he uses his detective vision to scan the rifle. As he does this, the TV's turn on, showing a recording of Joker.
"Yeah, but from where?" Hulk asks as he looks at the TV's.
"Well, look who it is. I haven't seen you boys for, how long has it been? Let's see, there was an asylum, some monsters, and oh, that's right, you left me to die."
"No we didn't. We locked you up." Hulk remarks.
"Now you probably don't remember it that way, but who cares?" Joker says, continuing his monologue. "You just need to worry about the bombs. Hurry up now. Clocks ticking."
"Bombs? What does he-?" Hulk asks, only for the TV's to display a countdown from five. Without a second thought, Hulk and Batman hurl themselves out the window, narrowly escaping as the explosives go off.
"Alfred, I've got a lock on the signal used to remotely control the sniper rifle. Joker's behind this." Batman says into his cowl as he perches on a billboard.
"Was there ever any doubt?" Alfred asks in an almost sarcastic tone.
"Not really." Hulk replies.
"The radio signal should lead us right to him." Batman says.
"Good luck, sirs." Alfred says as he ends the call.
-Later-
Hulk and Batman trace the signal, ending up at the Sionis Steel Mill in Amusement Mile, which no doubt was Joker's territory. Unfortunately, the place was crawling with armed goons, which made getting inside the Steel Mill problematic for the Caped Crusader, so he calls Alfred.
"Alfred, we need to find a route into the Sionis Steel Mill."
"Have you tried the front door, sir?" Alfred asks, to which Hulk snickers.
"Why didn't we think of that?" Batman jokingly asks.
"It was obviously too easy. Let's see. If all other access routes are locked down, the only way in would be...no...It's suicide." Alfred says.
"The main chimney." Batman says, already knowing that's the best way in. "Okay then." However, upon investigating, Batman realized the chimney was too small for Hulk. "Bad news. The chimney is too small for you."
"So I'm supposed to stay out here while you have all the fun?" Hulk asks.
"Pretty much." Batman replies.
"Grrr...fine." Hulk groans as Batman grapples up and into the chimney, landing on a tightrope that dangled above the smelting metal. A few minutes later, Batman crawls through the grating underneath the floor, overhearing Harley talking to a large group of thugs from a raised office.
"I promised you some entertainment, right, boys?" She asks, to which they loudly cheer in answer as a medic sits on her knees next to Harley.
"Please...I did my best." She pleads.
"Well, you should have tried harder." Harley says before kicking the medic down to the ground floor. "That useless quack has failed Mister J. And you know what that means." Harley says as the goons chant 'Kill' over and over. "Oh, I think we can do better than that. Get in here, Mister Hammer!"
A large, one-armed man wielding a steel sledgehammer walks in and continuously slams the large weapon downward, barely missing the cowering doctor, who ends up in a corner, trapped.
"No! Please!" She begs hysterically. Right as Mister Hammer is about to score a direct hit on the doctor, Harley Quinn calls out, making him stop.
"Stop! Change of plans. Take her to the smelting chamber. Mister J wants this one to suffer a little longer! The rest of you, get out! You know what to do! Go get the snowman and bring him back here right now! He's going to pay for screwing over Mister J. I need him back here like yesterday!" Harley says as Mister Hammer drags the doctor away while the vast group of thugs leaves the Steel Mill, running straight into Hulk, who grins as he attacks the thugs, making sure to hit them lightly as he fights them, managing to hold the massive door open long enough to get inside as it slams shut behind him.
"Found my own way in. The easy way." Hulk remarks as he helps Batman deal with some spare goons. "Need me for anything else?" He asks, to which Batman shakes his head.
"Not at the moment. I'll contact you if I need help." Batman says as Hulk nods before forcing the front door open and leaves. After that, Harley peers down from the office.
"What the hell are you doing here? You're supposed to be dead!" She complains. "Where's the other one? The handsome one with all the green?"
"I don't need him to take you down, Quinn." Batman says in response.
"Whatever, Bat-Brain. Well, there's no way you're getting to Mister J. We're safe and sound up here. You can just stay down there and burn!" She says meanly as she walks away, the door closing behind her, keeping Batman from entering.
"Alfred, Joker's holed up in the manager's office in the Steel Mill."
"I'm sure you'll find a way, sir." Alfred says in reply.
"Of course I will. Some of his goons dragged a doctor away. I'm going to find her first, then deal with Joker." Batman says with determination as he hangs up, heading off to save the doctor.
-Smelting Chamber-
Batman enters a large room and turns on his detective vision, highlighting several armed goons and the doctor, the latter of whom is tied to a chair while the goons walk around the room, clearly expecting Batman. A few minutes pass as Batman makes short work of them, silently knocking them out cold and hanging some of them uprise down from heightened structures as he unties the doctor.
"Can you stand?" Batman asks as the doctor stands up.
"Give me a second." She says as she rubs her wrists. "What's wrong with these people? That crazy woman thinks I can fix the Joker."
"What's wrong with him?"
"There's something in his blood, some kind of toxin. It's killing him." She says as she looks at him. "He blames something called 'Titan'."
"Titan?" Batman repeats in surprise.
"Yeah. Do you know what he's talking about? He kept going on about his last laugh."
A mannequin of Harley with an enlarged bobble head chimes in on the conversation. "Uh-uh, doc. Isn't that information supposed to be confidential?"
"What is happening? How does she know what we're saying?" The doctor asks, slightly alarmed as Batman notices the light of a welding torch behind the door he used to enter the room. The two were now trapped in the room. "Well...at least they won't be getting in. We're safe, right?" She asks Batman, who approaches a glowing canister and pulls out a small piece of technology. "What the hell are you doing?"
"I need to get out of here. Someone needs to stop Joker." He says in answer as he fuses the technology to one of his gadgets. "Stay here. You'll be safe." He says assuredly.
"You sure?" The doctor asks, not really feeling a vibe of safety.
"If you hear anyone coming, hide. I'll be back." Batman says as he leaves.
-Manager's Office Area-
Harley opens the door and looks down at Batman, noticing the new gadget he built. "How did you get out of the smelting chamber, B-man? What did you build? Tell me, tell me!" Harley says like an impatient child before hearing Joker call out to her.
"Harley! Get back in here! I think I'm dying!" Joker said hoarsely.
"Now see what you've done! Scram, B-man! Coming, Mister J!" Harley says as she runs out of Batman's view, once again shutting the door.
Batman shakes his head as he uses the gadget to electromagnetically pull a crane hook back and launch it forward, making it latch onto the door as he reverses the polarity of the electricity, tearing the door off the office.
"What's wrong with you, B-man?!" Harley exclaims at the destruction. "You come into Mister J's home and start smashing it to pieces? Don't you know he's sick?!" She asks as Batman grapples up to the doorway, only to be kicked in the face by the one-armed man from before, making Batman fall back down as the large man jumps down to ground level to fight him. "Surprise, B-man! Meet Mister Hammer. He's going to teach you some manners!" Harley says loudly as more henchmen join the fight. Even with Mister Hammer backing them up, they're no match against Batman's superior skills as he takes them all down, including Mister Hammer. Seconds after that, Harley cries out as Batman can hear the faint sound of a flatlining EKG. "No!" Harley cries out as Batman grapples up to the office, finally seeing Joker, who is sitting limp in a wheelchair with a breathing mask on his face. "Why, why, why, why, why?!" Harley asks as she sobs at the lifeless body of Joker, before looking at Batman with intense hate. "You!"
"Get out of my way, Quinn." Batman says as if he has no time to deal with her melodrama.
"Leave us alone, B-man...Just leave me with him..." Harley pleads pathetically.
"I said move!" Batman says sternly as he quite harshly pulls Harley away and tosses her onto the hardwood floor as he scans Joker with his detective vision, his life signs labeling him as deceased.
"This is all your fault! You've hounded him for years, beat him to a pulp, and for what? Why are you so mean?!" Harley asks coldly, to which Batman simply looks at her and doesn't say anything.
Two seconds later, Joker jumps onto Batman's back and presses a rebreather to his face, making him inhale a green gas that disorients the Caped Crusader. "Surprise! Hehahahahaha!" Joker cackles as Batman pushes him off. "You fell for the old 'fake Joker gag', Batman!" Joker says tauntingly as Harley readies a metal baseball bat.
"Batter up!" Harley says as she whacks Batman several times before finally hitting him right in the face, knocking him out cold.
-Author's Note-
I kind of struggled on whether or not to have Hulk stay in the Steel Mill until I realized that Harley would probably be unable to knock Hulk out as well, so I wrote in Hulk leaving to wander Arkham City.