Plagality is a pretty deep musical concept within tonality, though -- you have to deeply grok counterpoint and tonal harmony first, and then you can start to hear and perceive different aspects of tonal gravity.

For plagality, it's the "dark side" of the tritone, specifically with iiø or iv6.

Start putting together that tritone with the major standards of vii° or V7, and you're really rolling with the joy of traditional harmony.