The citizens of Gwangju lead a relatively peaceful life, until one day the military takes over the city, accusing the residents of conspiracy and claiming that they are communist sympathisers preparing a revolution against the current government. Seeing as the soldiers beat defenceless people, mainly students, to death, the citizens are in for retaliation and form a militia.
Even though it seems they have nothing in common, both films talk about the Gwangju Massacre.
May 18 is about the massacre from start to end and is mainly from the rebel's point of view.
Peppermint Candy talks about part of the massacre and the aftermath of it, but from the point of view of a man both when he was a young soldier, and later when he became a policeman.