The main thread or storyline of a talk or story; the principal point or main line of events.
話が脱線したが、本筋に戻そう。
The conversation got off track; let's get back to the main point.
小説の本筋は家族の再生だ。
The main thread of the novel is the family's renewal.
映画の本筋はシンプルだが、演出が巧みだ。
The film's storyline is simple, but the directing is skillful.