The Riverdale water supply is infused with an age-reversing chemical, and suddenly, all of the town's adults are turning into teenagers...including Mr. Lodge and Mr. Weatherbee, who are set to compete in a drag race, and Miss Grundy, who has an amorous eye for Reggie.

When Miss Beazley discovers Dilton's newest formula in the school fridge, she accidentally mistakes it for the secret ingredient to her tapioca pudding -- and once it's a part of the recipe, the pudding grows, and grows, until it's large enough to devour everything in its way! With Miss Beazley, Miss Grundy, and Betty in the beast's belly, it's up to Archie and Reggie to set aside their rivalry in order to work together to stop the pudding's rampage.

Archie is trapped alone in a department store for Christmas with a monster while everyone else tries to remember the spirit of Christmas.