Underplot and Collusion

• (n.) A clandestine scheme; a trick.
• (n.) A series of events in a play, proceeding collaterally with the main story, and subservient to it.

noun

• Secret agreement.
• Agreement on a secret plot.
  • synonyms: connivance

• (n.) An agreement between two or more persons to defraud a person of his rights, by the forms of law or to obtain an object forbidden by law.
• (n.) A secret agreement and cooperation for a fraudulent or deceitful purpose; a playing into each other's hands; deceit; fraud; cunning.

Collusion is a secret, deceitful agreement by two or more parties to defraud others.