Idioms and Phrases with plot thickens, the
plot thickens, the
Circumstances are becoming very complex or mysterious. Today this term is often used ironically or half-humorously, as in His companion wasn't his wife or his partner—the plot thickens. Originally (1671) it described the plot of a play that was overly intricate, and by the late 1800s it was used for increasingly complex mysteries in detective stories.