Steven Soderbergh and Matt Damon clearly had a good time making “The Informant!”—down to the exclamation point—and their enjoyment proves infectious. This is a movie of chuckles rather than belly-laughs—in many respects it’s like an American version of an Ealing comedy from the fifties. But they really add up.

You can think of the script by Scott Burns, based on the non-fiction book by Kurt Eichenwald, as a twisted comic variant of “The Insider.” Matt Damon, chunked up considerably and owlish behind glasses, plays Mark Whitacre, an eager young vice-president at Archer Daniels Midland, the agribusiness giant headquartered in Decatur, Illinois. A biochemist, he’s heading a project involving lysine, a food additive the firm hopes will increase yield—and profits.

But when his experiments repeatedly fail, Whitacre comes to his COO Mick Andreas (Tom Papa) with startling news—he’s been told that a Japanese firm has a mole in the plant who’s sabotaging the work, whom the informer offers to name for a ten million dollar payment. That prompts Andreas to call in the FBI instead, and Agent Brian Shepard (Scott Bakula) arrives at Mark’s house to tap his business line, on which the tip supposedly came in.

But Whitacre takes the opportunity to inform Shepard that ADM is involved in massive price-rigging with its competitors, and quickly agrees to work undercover for the Bureau, secretly taping his boss Terry Wilson (Rick Overton) as they travel to conferences to work out the details with other lysine producers.

But that’s only the beginning of the strange tale, because though Whitacre seems like a quirky, high-strung guy from the very first moment we met him, the extent of his peculiarity grows increasingly evident as the plot thickens and he becomes more and more erratic and unreliable, reveling in the role of secret agent in a way that has his government handlers aghast. The means that Burns, Soderbergh and Damon use to reveal his oddity is simple but simply dead-on—throughout Whitacre delivers interior monologues, disconnected ramblings replete with ridiculous factoids and weirder musings that are completely unrelated to what he’s actually doing at the time. As this stream-of-consciousness nonsense piles up, one gets the distinct impression that the guy’s off his rocker.

But that’s not all. As the case drags on, Whitacre’s stories change and turn on themselves, and his delusions become ever more astronomical. Eventually it’s revealed that though ADM has indeed been involved in price-fixing, Mark’s been up to shenanigans of his own, and he’s taken the FBI for a ride. Eventually he needs lawyers of his own, and he’ll always keep even them in the dark until the last possible moment.

There’s lunacy at every turn in “The Informant!”—in Whitacre, of course, but also in the bosses at ADM and the well-meaning but inept people in the FBI and the Justice Department. But the picture doesn’t hit you over the head with it. It ambles along loosely, relying on the absurdity of the situations themselves, Damon’s inspired turn as the brazen Whitacre, and the old-fashioned goofiness of Marvin Hamlisch’s score to generate more and more smiles as the complications escalate. The supporting cast make their contributions without exaggeration, with Bakula especially winning as a fellow who finds himself completely overwhelmed as his source’s fibs multiply. Both Tom and Dick Smothers make cameo appearances, but without the “look at me!” attitude that would break the carefully-maintained mood of hyper-controlled farce.

“The Informant!” also looks right, with the use of the actual locations giving everything an authentic feel and Doug Meerdink’s production design, the art direction by David E. Scott and William Hunter, the sets by Dawn Brown Manser, Jane Wuu and Dan Clancy, and the costumes by Shoshana Rubin unobtrusively reflecting the period. And the plain, unfussy cinematography of Peter Andrews (Soderbergh’s nom de camera) is similarly self-effacing.

In an era when successful movie comedies seem to require lots of flatulence and four-letter words, “The Informant!” is as much an oddity as its protagonist. It works on the funnybone gently, ratcheting up the level gradually in a way that requires a bit of patience of viewers. But like the Coen brothers’ more humorous outings, it’s consistently amusing in a quietly ironic way. And coming from Soderbergh, whose “Erin Brockovich” told a similar story of corporate skulduggery with an earnestness typical of the genre, it comes across as wittily self-referential, too. In its unforced, loopy way, this is one of the funniest pictures of the year.