original story (as opposed to a sequel, spin-off, etc.); main story; main part(s) of a story
this volume
the movie (as opposed to the trailers; at a movie theater); feature film (main item in a cinema program)
movie based on a TV series; movie version of a TV series
本編を見てから続編を楽しんだ。
I watched the main story and then enjoyed the sequel.
映画の本編が始まる前に予告編が流れた。
A trailer ran before the main feature of the film began.
本編に登場しないキャラクターが外伝に出てきた。
A character who does not appear in the main story appeared in the spin-off.