Villain of the week

"Villain of the week" (or, depending on genre, "monster of the week") is a concept that describes the format of certain television shows. As many American adventure-themed TV shows air episodes weekly, there is usually a new, temporary antagonist to forward the plot each week. The main characters usually confront and vanquish these characters, often leaving them never to be seen again.

See Witch Hunter Robin, Charmed, Sailor Moon, Jackie Chan Adventures, and Teen Titans for examples.