At one point, Henry Adams makes an emergency call to Batman asking for help. He tells him that Harley Quinn and her gang have attacked the quarantine cell and could break through the door at any time. So Batman quickly heads to the quarantine cell, because he doesn't want any Joker infected to escape from here.
Approaching the quarantine cell, Batman sees a large gate closed, which can only be opened by Harley Quinn's voice. So Batman uses his voice synthesizer to imitate Harley Quinn's voice and tell the guards to open the door. Just as the guards are about to open the door, Harley Quinn appears. She scolds them, saying that Batman is trying to fool them by imitating her voice.
Batman realizes that this plan will not work anymore. So he sends Robin ahead. Seeing Robin alone, Harley Quinn's men start laughing and say that Batman left him alone and ran away. Then Robin deliberately provokes Harley Quinn. He says, "Batman thinks I'm enough to catch you, so he left." Hearing this, Harley Quinn's pride is hurt. Angrily, she orders her men to open the gate and capture Robin. This is exactly what Batman wanted. When the gate opens, everyone surrounds Robin. At that moment, Batman jumps out of the tunnel below.
Then a great fight begins. Batman and Robin first knock Harley Quinn unconscious. Then they jump on the rest of the criminals. There were also two big "big men". They are not affected by ordinary punches, but Batman and Robin fight together and defeat them. During the duel, sometimes Batman throws the enemy into the air and Robin hits him with a stick, sometimes Robin holds someone and Batman punches him from the front. In this way, they knock all the criminals unconscious.
After the fight, Batman lifted Harley Quinn onto his shoulders and Robin took Christina Bell to the quarantine cell. They thought everyone needed to be treated quickly.
But when they enter, they see a horrific scene. The three infected they caught earlier are all shot dead. Then Batman realizes that the real culprit is Henry Adams. He has been pretending to be healthy, but in fact the Joker's influence is fully at work inside him. Henry reveals that he killed them all himself because he doesn't want there to be too many "Jokers." According to him, there will only be one Joker—and that Joker will be Batman. He reveals to everyone that Batman also has the Joker infection in his body. He then shoots and kills the newly infected.
Then Henry tells Batman to enter the quarantine cell. If Batman doesn't agree, he threatens to kill Harley Quinn. As Batman is about to enter, Henry suddenly laughs and says, "If you're alive, it'll be fun." Saying this, he shoots himself in the head and commits suicide.
The incident happens so quickly that it's over before anyone realizes it. Robin is shocked to learn that Batman is also infected by the Joker. He then asks Batman to enter the quarantine cell and get treatment, as this may be the only way to save him.
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