Duncan is an ordinary guy who is burdened by stress brought on by his boss, wife, mother, and an assortment of other individuals. After the stress piles on past control, Duncan begins to undergo terrible stomach problems. Eventually, Milo emerges from his behind and starts to rampage. As seen in the trailer, a coworker informs Duncan about possibly deleting an important file. Said coworker is attacked and murdered by Milo who leaves the corpse with the appearance of an animal attack. It is also shown attacking others, such as his boss and wife.
Over the course of the story, it is shown that Milo is a demon embodying Duncan's inability to handle his stress or his repressed anger. Thus, to prevent further murders, Duncan must bond with the creature and, by extension, quell his own internal rage.