Bok-soon (played by Kim Go-eun-I) is mentally challenged. She's intelligent enough to make a living as a street peddler, but that's the full extent of her abilities. The young woman has trouble keeping track of anything but the most simple short-term reasoning, although she does easily understand the concept of love. Ironically, the titular monster of this film, Tae-soo (played by Lee Min-ki) shares almost the exact same basic personality traits. It's just that where a bad day for Bok-soon means screaming like a crazy person and conking people on the head, a bad day for Tae-soo means brutally and violently murdering everyone in the room.
A lot of weirdness ensues as these two people interact with a real world that they only barely understand with radically different results. The shifts between scenes are perplexing- in one stretch we go from discovering the terrible secret behind Tae-soo's pottery, Bok-soon lightheartedly preparing for an epic quest, people abruptly and comically getting lost in the woods, to a bizarrely misleading showdown at an isolated cabin