The Plot Thickens Meaning

(idiomatic) Used, often with humorous intent, to describe an increasingly complex or mysterious situation.

Example: 1672 George Villiers, 2nd Duke of Buckingham, The Rehearsal, Act III Scene ii:
  BAYES. Lo' you now, there he's off again.
  JOHNS. Admirably done i'faith.
  BAYES. Ay, now the Plot thickens very much upon us.
1703–4 Richard Steele The Lying Lover: Or, the Ladies Friendship Act II Scene i:
  Y. Book. Well said, Lad—and as Mr. Bays says, now the Plot thickens upon us, we'll spend our time as gaily as the best of 'em—and all of it in Love [...]
1888 Frank Munsey The Boy Broker; Among the Kings of Wall Street Chapter XX, p.143
  "His statement is wholly false," was our hero’s reply. "It was his miserable villainy that deprived me of my liberty, and kept me away from my work."
  Mr. Goldwin looked puzzled.
  "The plot thickens," said he. "Give me your story."
1973 "Watergate: The Plot Thickens" The Sun–Herald (Sydney, Australia) 1 April 1973, p.75.