Mark Whitacre (Matt Damon) is the man every nerd aspires to be. He’s a vice president in charge of bioproducts at a huge agri-business, has a loving family and owns a sprawling estate in southern Illinois, where he works. He seems genuinely content with his life, so it is surprising when he decides to blow the whistle on shady doings going on at his corporation.

“The Informant!” is loosely based on Kurt Eichenwald’s book of the same name, which was published in 2000, and tells the story of a schlubby, mustachioed paradox of a man who is both a wide-eyed innocent and a ruthless schemer. Under pressure from his flaky, guilt-stricken wife Ginger (Melanie Lynskey, “Two and a Half Men”), Whitacre approaches the FBI with information about price-fixing at the corporation.

From the very start, this high-ranking executive’s motivation for coming forward seems a little fuzzy, but Special Agents Shepard (Scott Bakula, “Star Trek: Enterprise”) and Herndon (Joel McHale, “The Soup”) believe that Whitacre is acting nobly and seize the chance to run a major investigation.

Whitacre thus becomes an informant for the FBI, and he approaches the task with child-like, often infuriating excitement. He envisions himself as a superspy, but in reality he is little more than a highly amusing space cadet. Some of the film’s funniest moments occur when he noticeably speaks into his wire while walking around the office and fiddles with hidden cameras in his briefcase during meetings.

Amazingly, Whitacre never gets caught by his colleagues during his two and a half years of whistle-blowing, and because of him the FBI uncovers some serious white-collar crime.

Although the FBI agents successfully build a case against the corporate thieves, they discover too late that Whitacre is not the most reliable witness. Excited by his job as an informant, he opaquely and recklessly shares the fact with family members, a few coworkers and even his gardener. He constantly feeds the agents and his own lawyer obvious lies.

Throughout the film, it becomes apparent that he has been embezzling from the company. Despite stealing from and ratting out the agri-business, Whitacre irrationally believes that once his obnoxious bosses are prosecuted he will be recruited to run the company. It turns out the FBI’s star witness has more than a few loose screws.

What makes “The Informant!” so unusual is that it could have easily been made into a corporate thriller like “Michael Clayton.” Instead, director Steven Soderbergh (“Ocean’s Thirteen”) turns Whitacre’s story into a fictionalized dark comedy about a bipolar man who fools everybody but who is so delusional that he only gets himself into a load of trouble.

Soderbergh wisely decided that it would be futile to dig deeply into Whitacre’s warped, enigmatic motivation.

The story is so bizarre, and Whitacre is such a relic of the ’90s, complete with hairpiece and massive cell phone, that the story is far more effective when played for laughs. The film is funny without any physical humor or dirty jokes, providing a refreshing alternative for those who feel Judd Apatow’s recent comedies have gone stale.

The movie strikes the right tone because of Damon’s comical yet convincing performance. Whitacre is narcissistic, but his folksy Midwest personality is entirely genuine. He is also a pathological liar, but his personality is disarmingly transparent. He clearly loves his family and his job, and honestly does not believe he has done anything wrong.

Much of the film’s humor is derived from voiceovers of Whitacre’s thoughts, which display both an innocent, goofy sense of wonder at the world and a “Seinfeld”-esque fixation on the minutiae of everyday life. Even after it is revealed that he has been telling them less than the whole truth, the FBI agents still try to help him because they like him and are worried about him.

“The Informant!” manages to be fun and entertaining while also telling an ironic and true story about a genuinely fascinating character. Even when Whitacre’s behavior gets increasingly embarrassing and outrageous, you will never want to take your eyes off those dorky glasses and mustache.

— Contact Joel Dobben