The plot thickens
Definition 1 of 1
Pro Tip 1/3Narrative Suspense
Use it when a story or event takes a surprising twist, increasing suspense or curiosity. 
After the alibi was proven false, the plot thickened.
Pro Tip 2/3Dramatic Irony
Works well when the audience knows more than the characters, creating a sense of drama. 
She smiled at her foe, unaware of the secret; the plot thickens.
Pro Tip 3/3Everyday Complexity
Can be used humorously in daily life when complications arise out of ordinary situations. 
When both siblings claimed the last cookie, the plot thickened.