습도가 높은 어느 불쾌한 여름날, 코로나로 인해 존폐 위기에 놓인 영세한 극장에서 퇴물 영화감독 이희준의 신작 시사회가 열린다. 극장 사장이 정부 정책을 핑계대며 에어컨 가동을 거부하자, 더위에 지친 관객들의 불평과 예상치 못한 진상들의 공격이 거세지면서 알바생 찰스의 인내심도 바닥나기 시작한다.
Big Baby tells the story of Adam Lewis, a successful horror screenwriter struggling for inspiration for his latest script. After a graphic and realistic nightmare of a hulking man dressed in a baby mask and onesie who axe murders his girlfriend Kate in the middle of the night, Adam gets the inspiration he needs for his new screenplay. Excited about the direction his story is taking, he starts losing himself in his script. Things are better than ever for Adam and Kate until “Big Baby” starts appearing in real life and tormenting and killing victims fueled by his own revenge. Characters from Adam’s script begin to pay him visits pleading for their lives, and he quickly realizes he holds their fate in his hands. Power and fear completely consume Adam until his girlfriend Kate is terrified of the man she once loved.