Villain of the week

A "villain of the week" or, depending on genre, "monster of the week" (MOTW) or "freak of the week", is an antagonist that only appears in one episode of a multi-episode work of fiction, such as a television series.

The expression "monster of the week" was first used during the production of the 1960s series The Outer Limits. However, the origins of the concept predate this.