What’s the scam?

What's the scam?

What's going on? What are you trying to pull? Said when one is suspicious of someone else. A: "Why would you be afraid of talking to the police if you haven't done anything wrong? What's the scam, Mr. Thompson?" B: "Nothing, I swear! Police officers just make me nervous, that's all."
Farlex Dictionary of Idioms.

What’s the scam?

and What’s the deal?
interrog. What is going on around here?; Explain what is happening or what you are doing. What’s the scam? What’s happening, dude? I gave you a twenty, and you give me five back? What’s the deal? Where’s my other five?
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • cut (one) off at the pass
  • cut off at the pass
  • cat around
  • not trust (someone) as far as (one) can throw (them)
  • not trust someone as far as you can throw them
  • not trust (someone) farther than (one) can throw (them)
  • on the watch
  • on the watch (for someone or something)
  • something is rotten in (the state of) Denmark
  • something is rotten in the state of Denmark