The Joker says "better make it ten [minutes]", before heading up Gotham Cathedral. Exactly 10 minutes later, the helicopter flies into shot to get him. See more...
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Plot hole: The Joker publicly announces on television that he will be throwing a huge parade through the streets of the city. So this would be a great opportunity for the police department to apprehend the Joker and his henchmen. Instead, when the parade takes place there is not one police officer seen anywhere.