Batman remembers his first memory, playing outside Wayne Manor with his parents and Alfred. What he remembers most vividly is their laughter.
In Gotham, now, Batman is on the highway, chasing a truck which carries rowhouses, which he and Alfred know is a ploy to smuggle something outside the city. An informant has told Batman that these people hide three to four tons of stuff within the trucks' flatbeds, covering up the operation with the removal of historical rowhouses to make room for new condos. Other voices on the streets say that this company, "Happy Trails", was seen entering the city morgue, so Batman suspects they are taking bodies outside Gotham. He manages to subdue the thugs protecting the main rowhouse and to enter its false bottom to reveal a casket. To his and Alfred's surprise, inside it is... Bruce Wayne.
At the Gotham Morgue, Batman disguises himself as a coroner working there to be able to analyze the body. While Alfred is too distressed to keep concentrated, Batman checks that his DNA matches the corpse's, effectively confirming that he may be a Bruce Wayne from a parallel universe. Upon inspection, though, he also notices that the body lacks his most recent scars, that he had a daughter, he was married and that he has pronounced smile and laugh lines on his face: in one word, he was happy. Getting in the car outside, Bruce tells Alfred that he knows where they need to go.
At Arkham Asylum, two guards are killed in cold blood by an armed-to-the-teeth Batman, who then proceeds to open the door to the Joker's cell. The Joker asks him if he came to kill him, but the armed Batman replies that he didn't. Behind him, the Batman Who Laughs adds that he is, before plunging his scythe in the Joker's head.
At the GCPD HQ, Jim Gordon tells Batman that he didn't light the signal because the employees at Arkham are currently scared by him, even though deep down they know that he wasn't the one killing their friends. Batman explains to Gordon who the Batman Who Laughs is, and that his accomplice, the one they are calling "The Grim Knight", must've come with him from the Dark Multiverse but didn't come out before. Batman still can't connect the dots of Who Laughs's plan, so he leaves, looking for "help".
It turns out that the Joker knew what was coming for him, so he swapped places with one of his slapstick men and ran before Who Laughs might kill him. With a complex series of word plays on the victim's fake names, Batman infers that the Joker wants to meet him in the place that would make him the happiest: the Batcave.
Back at the Batcave, Batman orders Alfred to undo the Waterway Security System, so that anyone could enter through the tunnels. As he obeys, the Joker emerges from the water, pulling a gun against Batman and Alfred. The Dark Knight tries to explain they are on the same side this time, but the Joker pulls the trigger, which happens to shoot backwards, in his own chest. Batman runs to help him, but that was the Joker's plan all along: just like Who Laughs's Joker, he had a toxin in his heart that activates the moment he dies. The Joker explains in his dying breath that to defeat Who Laughs, Batman must become like him. As Alfred tries to understand what is going on, Batman goes mad, laughing maniacally and turning pale.
