When the Joker announced that he would rule Gotham City at midnight, the police began a massive evacuation of the city. However, the Joker implied that the tunnels and bridges were rigged with explosives, forcing the police to evacuate using ferries. The last ones to be loaded before midnight were the Liberty, which was carrying civilians, and the Spirit, which was carrying criminals. Commissioner Gordon wanted to get some criminals off the island, as he did not want them to take part in whatever the Joker was planning.
At 11:30, both ferries' engines stopped and both crews discovered massive amounts of ammonium nitrate in the engine room, rigged to blow. The Joker announced that if anyone attempted to get off, he would destroy both boats and that at midnight he would destroy both. Both boats were given a detonator to destroy the other one. If, one boat chose to destroy the other one, he would let the surviving boat go. The criminals on the Spirit, afraid of dying and missing a chance at reform, begged the prison guards to destroy the Liberty. The guards held them back with shotguns. One criminal, Ginty, walked up to the prison foreman and asked to be given the detonator, as he knew how to take a life. After receiving it, he threw the detonator into the harbor. At the same time, the people on the Liberty refused to kill the criminals, even after voting for it.
It can be assumed that after Batman subdued the Joker, everyone on the Liberty and Spirit were rescued.