Peripeteia is a turning point in a story song’s plot.
It’s a pivotal moment in which fortune reverses for a character or group of characters. The turn may be favorable or decidedly unfavorable, and it can occur either due to external circumstance, a personal revelation, or—most powerfully—both simultaneously.
To ensure that the story is believable and satisfying for listeners, a songwriter should take care to set up this turning point properly with previous action, making sure that the peripeteia not too sudden and unexpected. This can make the difference between a believable turning point and a clumsy deus ex machina.
Peripeteia can technically occur anywhere in a song, but an especially natural place for it to occur is in a bridge, where the musical shift calls the audience’s attention to the game-changing event happening in the lyric.