The First Laugh

Joker's plan involves the children. He will round up his psycho clowns, to serve as his lieutenants, and commit various crimes that make it appear that he is toppling Penguin and Black Mask, as well as raising his public awareness, such as the Joker fish. He will gain coverage in the media for his increasingly heinous crimes, which will inevitably be seen on the news by children. While on the surface, they are committing Clockworkian random acts of violence, attacks on lawful and illegal institutions, and possibly creating an empire, the reality is his master plan is to use children, the only true innocents, to willingly suicide bomb mass areas and significant structures. Why? To him, he and Batman, through the grand nature of their intellect and actions, are the only ones capable of cutting above the rest of mankind. The Joker believes mankind is fundamentally bad because it can be bad, and thus it is not worth the effort of saving, thus it irritates him to see Batman proving his philosophy incorrect. It defeats his self-defined purpose. So to stop Batman from helping people and fighting evil, he will prove to him that even the most innocent or good-natured people are easily capable and willing to commit atrocities.
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One of these children, specifically, is Jordan Hill, son of notably corrupt Mayor Hamilton Hill, almost as if to prove that if the father is evil the son is evil. Another of his targets is Barbara Gordon, daughter of rising cop Jim Gordon, however, Barbara is resistant to Joker's plan.
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Joker is deadly in one way in that he doesn't write down his plans at all. It's all in his head. That's why a 25 year old can pull it off. He's brilliant in the vain of MGK. 
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What does a guy like Black Mask care about? What's a sore spot for him? For one, he measures his worth by his inherited influence and money, which he burned through in The Bounty, in part from absurd Belfortian adventuring and expenditures, but also in the stake against Batman (didn't go to prison because of his mask use) so he's already insecure to have been bought out by Bruce Wayne, especially considering Wayne left the city. 
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So using his chemical genius, J figures out a way to use common household Janus Cosmetics products to make explosives, and advertises this to fringe youth via message boards, Anti-Social Media, etc to act as an INFANTry for chaos. This causes Janus stocks to crash and Roman has no choice but to sell out to Wayne.
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This gives Black Mask a vendetta against the clown (sidenote -- just as Batman goes to the graves of his parents for their blessing, Black Mask went to graves of his parents for their scorn; to carve his mask)
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Shortly before the conclusion of the events of Dark Moon Rising, chalk white fish with rictus red grins show up in Gotham's fisheries. Captain James Gordon, soon to be appointed Major in conjunction with the appointment of Peter Grogan as Commissioner, is at a loss as to how to contact Bruce Wayne for help with the case. He then questions is value as a police officer if he has to rely on a vigilante. Monitoring the situation, Batman contacts Gordon to reveal to him his theory. The two agree that it is connected to the mysterious Joker playing card mailed to the GCPD about a week ago. Batman has performed an autopsy on the fish, confirming it to be a real fish that has been chemically altered with chemicals available at Ace Chemicals. As to the meaning of the threat, both men are at a total loss.
Across
Joker breaks up Peter Grogan's announcement?
Black Mask's men “kidnap” Joker (plants or his willingness?) and Black Mask tortures Joker. Joker takes it and laughs his way through it. Batman interrupts to save Joker, being the only time he knows his location, and a three way fight breaks out. Black Mask's men interfere and Batman escapes, taking Joker with him. 
Batman loads Joker into the Batmobile and they talk or something. Joker instructs Batman to turn his police scanner on. There's a child wearing a suicide vest at a movie premiere in Burnley. The police can't enter because of the threat that it could go off. Batman has no choice but to rush over there with Joker and drop him off with the officers there. On the way Joker gives his spiel that Batman should give up hope on trying to save man as he is inevitably bad. And they have a dialogue, two viewpoints, two knights with lances jousting. 
Batman makes it to theatre and turns Joker over to the officers, having the Batmobile drive away so they can't do anything to it (also freeing the Joker from its 'fensive systems.) Batman enters through an air duct, surveys the situation; the child, no older than ten, standing in front of the screen as the image projected, threatening to kill the audience if anyone stands or enters. Batman drops down and subdues the child (unlike the later agents this is not the child of a prominent citizen connected to the next location, so as not to make the plan obvious,) and brings him out of the theatre (the lobby has not been evacuated, as per the Joker's instructions, the boy sent one viewer out tell the police not to evacuate the lobby, checking via FaceTime to see they didn't) as Batman walks through his suit scrambles the phones of people recording. When he returns to the police, he finds that Joker is gone, having killed the officers and escaped. 
Batman takes the boy to Gordon (note: Batman couldn't have really dropped Joker off at a smaller precinct because there wasn't really time, for one, and for two, he was a such a high threat) at the precinct, and when they interview the boy, they find that he is highly radical at this point and not brainwashed on a surface level. They have to call in Batman to try, through what they think will be force (true desperation) and Batman is initially rough with the boy, but, shockingly, softens on him and is able to extract the information. But it's just what Joker gave him knowing he'd give to them. There will be more bombers. All children. For not even children are pure. 
Batman and Gordon try to brainstorm the next target. It's still more or less random at this point, but they're fairly quick to realize that Hamilton Hill's funding function is a serious possibility. Batman races to get there. There, Jordan Hamilton is revealed to be the bomber. Batman arrives for Jordan to press the button, but it turns out the button doesn't active the bomb. Batman has to forcefully interrogate Jordan, who breaks into tears, admitting that the next bomber is Roland Daggett's daughter, and that she'll blow up the Daggett plant.
Before the bomb squad arrives at the Mayor's party in Bristol, Batman is able to make it to the Daggett plant. At the plant, Batman is able to confront a crying Abby Daggett as she prepared to blow the plant. He stops her and works to disarm the bomb. She tricks him and attempts to stab him, but he deflects it and punches her in the face. As he opens the mechanisms, he finds a note for him, thanking him for playing along. “HA HA HA” plays on the LCD screen and Batman realizes what's going on, and escaped with Abby as the plant explodes. 
Simultaneously, the bomb at the theater and at the Mayor's estate explode, having been set off remotely by the Joker. He never entrusted the kids to blow them up, let alone at the right time, but this has all been his plan. Batman would think he stopped the bombing and he'd be forced to go on to the next one, but most likely escape. Batman meets back with Gordon at the precinct, and Gordon informs him that the Mayor and his son survived, but that an estimated fifty still fell casualty to the explosions. Barbara Gordon walks into the precinct, causing high tension, but she reveals that she does not have a bomb but Joker contacted her. He attempted to recruit her but failed, but gave her his location at that time on the condition that she walk in after the other explosions occurred. Joker held her captive to ensure the success of her timing. Batman silently notes her bravery and Gordon thanks God she's okay, not only physically, but as a person and that he didn't fail her. 
Batman arrives at the Sionis Steel plant, where Joker is now torturing Black Mask. Batman fights the psycho clowns, and ends up causing Joker to drop Black Mask, resulting in his mask being burned onto his face by the molten steel. Suddenly, Penguin's forces arrive and open fire, as do Black Mask's. You'd think they would have arrived earlier, but I suppose that's the last place you'd look.
They all fight it out, and what ends up happening is Batman, Joker, and Black Mask fighting on the roof. Joker stabs Black Mask and sends him off the roof but he manages to catch a window and survive. Batman and Joker engage in an intense, ferocious fight, with one of Batman's blades scraping the bone on Joker's forearm. Eventually, Batman gets the upper hand and holds Joker over the edge. Joker muses,
“You've won, now kill me.”
“No. I'll never. There's always hope for you.”
“You mean to to tell me after all of that, you think I can be saved? ME? HA HAHA HA HAHA HA!!!” this is at the end of their first encounter and drives Joker to try to force Batman to kill him. The police helicopters arrive and capture Joker, and either Black Mask is taken to a hospital or his men rescue him.
Batman is left alone on the rooftop to reflect.
Laughing Fish opening
Prior fights don't give enough time for real dialogue…
Daggett's loss will inform him to take radical steps that lead to the creation of Clayface and further criminal activity, which before he didn't do so much. 
“They are more willing to be led to do bad than good”
This also contributes to the “relationship” between Barbara and Joker leading into One Bad Day and when she eventually breaks his fucking teeth.
This also gives ground to the returning Falcones and Maronis to see this town as being taken by the freaks, and thinking that they'd be welcome to retake it.
Black Mask still hires Joker in UTRH, a story all about people Joker has wronged, which is an excellent plot point.