coup de théâtre

1. A sudden dramatic turn of events in a play.
2. An unexpected and sensational event, especially one that reverses or negates a prevailing situation.

[French : coup, stroke + de, of + théâtre, theater.]

coup de théâtre [koo dĕ tay‐ahtr], a sudden, surprising turn of events that gives a new twist to the plot of a play. Typical coups de théâtre involve the unveiling of a disguised character or the reappearance of one assumed by the audience to be dead. See also peripeteia.

The noun has 3 meanings:

Meaning #1: a dramatic surprise

Meaning #2: a sensational bit of stagecraft

Meaning #3: a highly successful theatrical production