Gretel's boyfriend Ashton introduces her to a strain of marijuana called "Black Forest" which is produced by an old lady named Agnes in Pasadena. After Gretel exhorts him to get more, Ashton visits Agnes where he is drugged and realizes Agnes is a witch. She then proceeds to eat parts of Ashton's body and eventually sucks out his youth (which restores her youth in turn). Gretel and her brother Hansel begin searching for Ashton, but they are ridiculed by the police and the trail ends with Agnes, who reveals nothing.
Meanwhile, local drug kingpin Carlos intimidates Agnes' dealer Manny into giving him the address of her house. Manny finds Agnes first to warn her about Carlos but is killed by her. When Carlos arrives, she easily dispatches Carlos and turns him into a zombie. The next morning, Gretel and Manny's girlfriend Bianca decide to infiltrate Agnes' house. Before they leave, Gretel emails Hansel that she is going to confront Agnes again.
While Bianca distracts Agnes through various means, Gretel sneaks into the basement where she finds the Black Forest crop, and the remains of the men Agnes has killed. Gretel leaves a trail of Skittles to help her find her way through the Black Forest. Eventually, Agnes sees through the ruse and the two girls are captured. Hansel shows up and is confronted by Carlos. Although Hansel defeats the zombified Carlos, he is knocked out by Agnes and placed in the oven room for cooking preparation.
Gretel and Bianca break out of their cage and stop Agnes right before she cooks Hansel, but Agnes manages to kill Bianca. During the struggle, Gretel pushes Agnes into the oven and locks her in. The oven explodes, causing the marijuana crop and house to burn down. Hansel and Gretel manage to escape.
As various first responders arrive at the scene, one of them picks up a cat strolling in the ashes into his van. The responder then screams in agony as he is slaughtered in his van. Agnes is now behind the wheel and drives away from the scene.
