In 1995, acclaimed poet Sidney Johnson accidentally interrupts his 12-year-old son Isaiah masturbating. He apologizes and reassures Isaiah that the act is natural, unaware that Isaiah was masturbating to a photograph of Sidney as a young man.
In 2009, during Isaiah's wedding to Marianne, Sidney's wife and Isaiah's mother Joan discovers Isaiah secretly performing oral sex on a distressed Sidney. Despite being disturbed, Joan tries to regain her composure. That night, while Joan is in the shower, Sidney leaves a typed memoir underneath Joan's pillow called
Cocoon Man: Confessions by Sidney Johnson. The memoir chronicles Sidney's experience of being incestuously abused by his son. Isaiah discovers it and tells him not to print any more copies. After a New Year's Eve party in 2010, Isaiah's erratic behavior begins to show as he lashes out at Marianne.
That night, while Sidney is listening to a self-help tape in the bath, Isaiah breaks down the door and rapes him. Joan hears Sidney's screams but turns up the television. The next day, Sidney attempts to leave the house with a secret copy of
Cocoon Man but is led into another confrontation with Isaiah, who
gaslights him. Sidney runs into the street in an attempt to escape, but is hit and killed by an oncoming van.
After Sidney's funeral, Joan decides to confront Isaiah, speculating that the abuse began 10 years ago during his prom night as she remembers Sidney crying for hours after it. Isaiah tells Joan that she is delusional and she calls him Sidney's killer, causing him to lash out and tell Joan that he loved Sidney better than she ever did. Isaiah attempts to strangle Joan, only for her to stab him with a knife. He then tries forcing her head into the fireplace but she stabs him to death with a
fire iron, throws the copy of
Cocoon Man into the fire, and sobs as she watches it burn.