Expression used here and there by some people
An expression originally used when something is introduced to the plot in a novel, movie, etc., to make it more complicated or interesting, but is now also used outside that context to indicate a set of circumstances has become more complex, mysterious, interesting, or difficult to understand.
"Remember I told you I keep finding rubber ducks at my doorstep?" "Yeah?" "Turns out the same thing is happening to my sister!" "Wow, the plot thickens"
Added June 2020 by amarens
13 out of 17 English speakers have heard this