Some odd years ago, the brothers Archibald and Roland Ironfist fought a brutal civil war for control of Enroth. Roland won, and had Archibald magically imprisoned as a statue. He married Princess Catherine of the allied kingdom of Erathia, and their son, Nicolai, became heir to the throne.
Three years ago, a strange force of devils appeared in western Enroth. Running rampant, killing people, and generally doing devilish things, they laid siege to the village of Sweet Water, hometown of the player characters. The party barely managed to escape with the aid of the sage Falagar, and Sweet Water was destroyed.
In response to the devils, King Roland mustered an army and headed off to try and stop them. However, an evil cult working with the devils had a traitor in Roland's army, who arranged a trap. The devils attacked, and Roland vanished.
The party, in the meantime, trained under Falagar near the town of New Sorpigal in an effort to get revenge for the destruction of Sweet Water. At some point during this training, they stumbled across an abandoned camp, where they found a set of letters linking the cult to the devils, and revealing the identity of some of the cult members.
Armed with these letters, limited supplies, and some basic adventuring know-how, it falls to these four people to stop the devils and the cult, and save the world.
Note: The linked intro video is not mine.