I think BtVS became a serial somewhere around NKABOTFD..
BTVS is sort of like Fringe and Firefly and Angel in that it was initially set up as an episodic series - ie. people catch and kill a monster each week, with the emotional relationship arc in the background. You might get a few serial heavy episodes, but most are stand-a-lone. But...gradually, it became more serial with just a few stand-a-lone episodes thrown in, until somewhere around S5 - they gave up on the stand-a-lone episodes all together and went pure serial. (Although there were a few scattered in there here and there - they tended to be weak episodes though - Doublemeat Palace is an example for S6) Did the same thing with Angel in S3-4 - gave up on stand-a-lone and went serial. (Because Whedon sucks at stand-a-lone, the weak episodes tend to be the stand-a-lone's in every season.) In S7...we do see a brief flirtation with stand-a-lone again with Lessons, Help, and possibly Him, but not really - you still need to know the back-story or you'd be completely lost.
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BTVS is sort of like Fringe and Firefly and Angel in that it was initially set up as an episodic series - ie. people catch and kill a monster each week, with the emotional relationship arc in the background.
You might get a few serial heavy episodes, but most are stand-a-lone. But...gradually, it became more serial with just a few stand-a-lone episodes thrown in, until somewhere around S5 - they gave up on the stand-a-lone episodes all together and went pure serial. (Although there were a few scattered in there here and there - they tended to be weak episodes though - Doublemeat Palace is an example for S6) Did the same thing with Angel in S3-4 - gave up on stand-a-lone and went serial. (Because Whedon sucks at stand-a-lone, the weak episodes tend to be the stand-a-lone's in every season.) In S7...we do see a brief flirtation with stand-a-lone again with Lessons, Help, and possibly Him, but not really - you still need to know the back-story or you'd be completely lost.