So let’s say you have a series of books, movies, TV episodes, TV seasons, etc.  Any given series of storytelling.  Doesn’t have to be linear in time.  Doesn’t even have to be about the same characters (a la romances about a number of sisters, etc.)  With or without an overarching plot.  Could just be “monster of the week.”

What’s the secret of having a good series?  All other things being equal?

Having the setting be the same but different.

Let’s look at Harry Potter as one example:

  • Book 1, Philosopher’s/Sorcerer’s Stone: Explore Hogwarts.
  • Book 2, Chamber of Secrets: Explore the dark/forbidden side of Hogwarts.
  • Book 3, Prisoner of Azkaban: How the wizarding world interacts with muggles.
  • Book 4, Goblet of Fire: The wizarding world is truly a world now, although it mostly just comes to visit.
  • Book 5, Order of the Phoenix:  That strange country, the past, which would like to repeat itself.
  • Book 6, Half-Blood Prince: How the bad guys see all of this.
  • Book 7, Deathly Hallows: Isolated from the castle/Final battle at the castle.

All of the books except the first half of Book 7 center around Hogwarts and its environs; none of the books are about the same parts/aspects/perspectives on Hogwarts.  And each of the plots influences/is influenced by the aspect of the Hogwarts setting that is invoked.

Take a look at other series that you like.  Not only are the settings probably not exactly the same, but the plots also vary a bit based on that aspect of the setting.