Okay, I've seen the movie six times, should be able to answer correctly....
Near the end of the film, the Joker has blown up Gotham General and escaped, taking with him a collection of hostages, while Dent has left the hospital to get revenge on those he thinks responsible for killing Rachel. Dent is confronting cops and mobsters, asking them about Rachel and deciding their fate with the flip of a coin (he shoots a mob limo driver and an old cop). He ends up kidnapping Gordon's family and holding them hostage in the building Rachel died in.
Meanwhile, the Joker has set up a "social experiment." Telling the people of Gotham on TV that they should leave the city, but saying the bridges and tunnels would be rigged with "surprises," folks all gather to leave the city in boats. At night, two boats finally leave, one loaded with citizens and the other loaded with Dent's captured criminals, because Gordon and the Mayor think they might be involved in the Joker's plan, and so evacuate them.
Suddenly, just after they leave the docks, the two boats stop, losing power. The crews soon find out that there are hundreds of oil drums in the ship's engine room rigged to blow, and each ship has been given a detonator. The Joker, watching from the higher levels of a waterside skyscraper, tells them that each ship has the detonator to the other ship's explosives. The ship that first detonates the other will live, but if they don't do it by midnight, both ships will blow up. Both ship's passengers are panicking, and the criminals become unruly while the citizens start to vote on whether or not to detonate.
Not to be left out, Bruce Wayne has used Lucius's sonar device on every cell in Gotham, to keep watch on all the city's inhabitants in order to find the Joker. Lucius agrees to operate and watch the cameras while Bruce takes on the Joker, but he decrees that it is too much power for one man, and will leave the company when they are finished. Later, when the Joker is talking to the shipcrews, Lucius discovers his location, and the Batman rushes out on his batpod to stop him.
Gordon and his officers have set up on a nearby building while two SWAT teams prepare to breach the building. Batman tries to convice him to wait, as the clowns in clear view of the snipers seem suspicious, but Gordon refuses to wait more than two minutes. Batman infiltrates the building and discovers that the clowns are restrained and they are actually hostages, but cannot contact Gordon. After he saves the hostages about to be sniped, Bruce goes throughout the building saving hostages from both the Joker's men and the SWAT team breaching the building (as this happens, the ships are decided whether or not to detonate). After all enemies are incapacitated, not killed, he makes his way to the Joker's level.
The Joker is standing near the edge of the building, overseeing the ships. He greets Batman and sends the dogs surrounding him after Batman, then begins to fight the Bat himself with fists and a metal pole. Cunningly and quickly, the Joker subdues the Batman and traps him under heavy piece of the still-in-progress building. As the clock strikes midnight, the Joker looks out to the ships but is suprised that they do not blow up. The Batman, resilient, asks what he tried to prove, explaining how the humans of Gotham are strong, and that the Joker is alone. Irritated, the Joker gets out his own detonator, arming it and asking the Batman if he told him how he got his scars. The Batman says "No, but I know how you got these." He shoots the sharp blades on his arm out, suprising the Joker and making him drop the detonator. The Batman then, in a show of strength, pushes the building piece off him and kicks the Joker off the building, who laughs madly all the way down. Bruce, however, catches him with his grappling hook, not allowing the Joker to break his rule.
Hanging in air from the building frame, stuck to the grappling device, the Joker explains how he corrupted Dent and brought him down to their level, and how this ace in the hole will break the spirit of Gotham completely. Batman leaves, and the Joker laughs exhaustingly, waiting to be captured by the SWAT team.
Gordon arrives at Dent and his family's location, and is immediately struck by Dent and forced to lay on the ground. Harvey explains what he's lost, and wants to make Gordon suffer, taking his son and putting a gun to his head. Batman arrives, asking Harvey to point the gun at the people responsible for Rachel's death. Harvey agrees, tosses his coin, and shoots the Batman in the abdomen. He tosses again, and does not shoot himself. Finally, he tosses again, after asking Gordon to tell his son it was going to be all right. Before he catches the coin again however, Batman tackles him and the son, falling over the side of the second story. He hangs by a hand from the side with the son while Dent falls to his death, and pushes the son up, but fal