Veteranen, William Tell (Oscar Isaac), lever af at gamble. Han opsøges af den unge og aggressive, Cirk (Tye Sheridan), som ønsker hans hjælp med at hævne sig på den tidligere oberst, John Gordo (Willem Dafoe). William ser en chance for at genvinde sin værdighed, hvis han lykkes med at forhindre Cirks destruktive planer og tager derfor på casino road trip sammen med ham. Rejsen sponsoreres af den hemmelighedsfulde La Linda (Tiffany Haddish), og målet for den umage trio er VM i poker i Las Vegas. Men i forsøget på at holde Cirk i kort snor, drages William ind i sin egen fortids mørkeste afkroge. The Card Counter er skrevet og instrueret af den legendariske filmskaber, Paul Schrader, som bl.a. har skrevet Taxi Driver og Raging Bull og som instruerede First Reformed (2017).
The film, written and directed by Schrader, follows William Tell (Isaac), a gambler and former serviceman who sets out to reform a young man seeking revenge on a mutual enemy from their past. Tell just wants to play cards. His spartan existence on the casino trail is shattered when he is approached by Cirk, a vulnerable and angry young man seeking help to execute his plan for revenge on a military colonel. Tell sees a chance at redemption through his relationship with Cirk. Gaining backing from mysterious gambling financier La Linda, Tell takes Cirk with him on the road, going from casino to casino until the unlikely trio set their sights on winning the World Series of poker in Las Vegas. But keeping Cirk on the straight-and-narrow proves impossible, dragging Tell back into the darkness of his past.