As the name implies, the Monster Of The Week is the bread and butter of a monster-fighting action series. I could list this as 2, 3, 5, 7, 8, 10-12... but as the show tries to ring the changes I'll show some different ways to do one. For now, this is the ordinary episode that the others deviate from, where the really important things are any resulting developments between the Cast, which are largely up to their players anyway.
All you really have to provide is a monster. These should be something you can imagine a weekly TV show being able to afford, for that matter. A subplot (which may also connect to what the monster is about) is good too.
These can involve variations on your standard monster like Dracula compared to regular vampires, classic archetypes like ghosts and killer cults, knockoffs from popular culture that you duly lampshade, scary things you thought up yourself. Grab some from here if you're stuck or use Scream Team as a visual guide or open another game's monster book.
Optional variants include the Minor Recurring Villain Monster, the Mystery Monster, the Creepy Monster and the TV-Friendly Eight-Strong Horde of Monsters.
What matters is that it threatens the peace and provides a decent fight, ideally with something interesting about it like a tactical option or a setting that could add flavour.
Their plans tend not to be very complicated - "get all up in the Hero's face until you get smacked down" is surprisingly popular - so you can always have them attack the Cast or kidnap one of the designated helpless victims if the players get stuck.
Mostly, MOTW episodes let the players talk in-character and air various issues without a serious danger getting in the way. If the MOTW itself is fun that's an added bonus. It's a dirty job but something's gotta do it.
Example: Don't Go There
The band have to arrange new digs, ideally including space for a decent practice room. Meanwhile, ninja demons trying to kill Billie. For a bit of added interest, they're trying to kill her in a peculiar way, like properly arranged duels to the death or something.
Another Example Because MOTWs Are So Common: Battle Of The Bands
A pretty obvious MOTW or possible recurring enemy-slash-irritant for a monster-hunting band. A rival band with a bit of magic of their own, which they use for their own self-aggrandisement. (Got a copy of The Magic Box? Turn to page 16.) This can escalate to a full-on ritual-disrupting fight, possibly partially through music and partially through violence in proper Scott Pilgrim style.