Two elderly sisters invite their wonderful nieces and nephews to a dinner party in celebration of the sisters' upcoming birthdays. The one nephew who is not invited is the ostracized black sheep of the family whose devil-worshipping activities have resulted in his being removed from the sisters' inheritance. The rest of the guests are merely putting in time; they are actually only waiting for their aunts to die, leaving them amply endowed via their respective inheritances. Unfortunately for all but the aunts, the nephew sends a party gift that turns the scene into a frolic of the macabre and ruins the party: under the gift's power, the aunts turn into cannibals and proceed to eat up all of their guests.

Due to its unusual subject and title and its graphic scenes of gore, Rabid Grannies is one of the most infamous titles in the Troma library.