One scene of Batman: The Dark Knight has the Joker, who managed to place bombs on two ferries: one carrying prisoners and the other carrying law-abiding citizens. The Joker has also given each boat the detonator to the other boat’s explosives, and threatens that if someone does not blow the other boat up then he will detonate the explosives on both boats. He is performing a twisted social experiment to see who would detonate each other’s bombs first.
The Joker is portrayed to think that society arbitrarily creates morals and that people follow those morals blindly. In other words, he believes that people are controlled by society and/or the social structures it creates. He sets up the “ferry experiment” to prove his point, expecting the law-abiding citizens to kill dozens of prisoners simply because society has condemned those people. The twist is that neither boat uses their detonator, which proves that those people were under no control but their own. This conclusion to the scene supports the idea that control is an illusion, and also helps Batman defeat the villain.