Christian 'Chris' Wolff, portrayed by Ben Affleck, is not your typical forensic accountant. Gifted with incredible mental calculation skills, he delves into the shady world of financial crimes, working for various criminal organizations. His assignments come through a mysterious woman's voice from an untraceable number. Instead of cash, Chris prefers unconventional payments such as rare comics, gold bricks, and famous artworks.
But danger lurks around him as he’s pursued by Raymond 'Ray' King, the determined director of FinCEN played by J.K. Simmons. King sees Chris as a threat, nicknaming him "The Accountant," and blackmails young data analyst Marybeth Medina into tracking him down. She risks exposure of her own dark past if she refuses to help him capture Chris before his impending retirement.
Chris’s childhood was marked by challenges, having been diagnosed with high-functioning autism. He was offered a chance to live at the Harbor Neuroscience Institute, where he formed a bond with Justine, the director’s mute daughter. His father, however, opted to raise him with his neurotypical brother Braxton, believing challenges were essential for Chris’s development. The strain of raising a child with special needs pushed his mother away, leading their military father to provide them with rigorous training around the globe, skills Chris now employs to safeguard himself.
When he receives his next job auditing Living Robotics, Chris meets the company's sharp accountant, Dana Cummings, who has uncovered alarming financial irregularities. Chris finds himself amidst a web of deceit involving the CEO Lamar Blackburn and his sister Rita. The company’s CFO, Ed Chilton, initially dismisses Dana's concerns, but Chris swiftly uncovers that $61 million has gone missing. After a tragic confrontation leads to Chilton's demise, Chris feels compelled to continue his investigation, only to be thwarted by Lamar's decisions.
Meanwhile, Medina pieces together clues about Chris's cover identities, which are inspired by famous mathematicians. Her investigation brings her to the accountant's covert office, ZZZ Accounting, as well as financial donations from Chris to the Harbor Institute. As danger escalates, Chris and Dana find themselves targeted for elimination, forcing Chris to protect her while revealing the truth behind the embezzlement—money that was funneled into affiliated companies to inflate stock prices.
As the plot twists further, Chris learns that Rita has met a grim fate at the hands of a hitman, revealing Lamar as the true villain. King and Medina uncover evidence of Chris’s alarming rise within the dark world of crime-fighting and his troubled past marked by violence. Ultimately, Chris's final confrontation leads him to Lamar's mansion, where he faces off against mercenaries and comes to terms with the estrangement from his brother, Braxton.
The reunion is tense but cathartic, ultimately leading to a newfound understanding between the siblings. In the aftermath, Chris sends Dana a token of appreciation—a Pollock painting, shrouded with another painting as a humorous nod to their earlier conversation. The film closes with a revelation that the guiding voice manipulating events is, in fact, a sophisticated computer system created through Christian's generous donation to the Harbor Institute, now used by Justine, who continues to communicate through its innovative technology.
