Scooby-Doo.Mystery.Incorporated.S01E11.The.Secret.Serum.

There is a vampire loose in the town, terrorizing the residents and stealing an odd assortment of loot. When the gang does some sniffing around, they discover an unlikely suspect - Daphne's mother! With things already strained within the gang, this latest blow tears them in half - girls on one side, boys on the other.