main thread; main plot; main point
Refers to the central storyline of a narrative, or the essential point of a discussion. Often contrasted with digressions or side issues.
The conversation strayed from the main point.
The main plot of this novel depicts family bonds.
Compound of 本 (main) and 筋 (line, thread, plot). The combination straightforwardly yields 'main thread' or 'main plot'.