There are towns where the border between our world and the world of monsters - properly called Legends - is thin. One of those towns is Darkmouth. There, our hero Finn is the son of the last remaining Legend Hunter - which means that one day soon he will be the last remaining Legend Hunter. Except, he's a bit rubbish at it. And in a spectacularly unfortunate coincidence, the terrifying leader of the monsters is in the midst of planning an all out attach on earth - in Darkmouth.