“Round up the usual suspects” is a famous line from Casablanca. One reason for its fame is its irony: how can suspects be “usual”?
The Usual Suspects tells the story in the voice of Verbal Kint, as he’s being interrogated by a police officer. Five individuals with criminal history were arrested in conjunction of a stolen truck of weapons. The lineup was a farce (as was in Casablanca) and they were locked up for a night. During that night, a plot was hatched to target corrupt cops. Of course, nothing happens by chance. We learn that these five were brought together by Keyser Soze because they are each indebted to him. They were forced to kill an individual to repay the debt.
This is a classic study of untrustworthy narrator. Can we believe anything that Kint says? Is there a real Keyser Soze. We, and the officers, are led to suspect that Dean Keaton is the mastermind behind this. This was the prior belief of the police and Kint cleverly and subtly feeds this suspicion.