The plot thickens as we find more clues.
the plot thickens
idiomUsed to describe an increasingly complex or mysterious situation.
Origin
first recorded 17th c.From the satirical play The Rehearsal. First attested in 1671.
I thought the mystery was solved, but then the plot thickens when a new witness appears.
As the detective uncovered a hidden connection between the suspects, he realized the plot thickens, revealing a conspiracy far deeper than he had initially imagined.