Screenwriter Mark Bellison (RICKY GERVAIS) lives in a world where no one lies. That's not a conscious decision, it's just that lying in any form, be that fibs, deceit, flattery or even fiction, simply doesn't exist. As a result, Mark's work for Lecture Films is penning factual scripts about historical events -- his covering the 14th century -- where the resultant movie simply features a person reading the script on camera.
Yet, his boss, Anthony (JEFFREY TAMBOR), isn't happy with his work, his personal assistant, Shelley (TINA FEY), hates working for him, and his work rival, Brad Kessler (ROB LOWE), thinks little of him. And since no lies exist, they simply state their feelings about him without any sort of social filters. It short, it's a brutally honest world.
The same holds true for Mark's personal life, when his best friend, Greg (LOUIS C.K.), sets him up on a date with Anna McDoogles (JENNIFER GARNER), but she immediately informs him that he's not anywhere near her type and no future together is possible since she wouldn't want fat kids with snub noses.
Things then go from bad to worse when Anthony fires Mark and his landlord gives him one day before evicting him. Desperate, Mark goes to the bank to withdraw what little money he has, but then something interesting occurs. With the bank's computer system down and unable to indicate his balance, something snaps in Mark's brain and he suddenly has the ability to lie.
Since everyone believes everything that's said at face value, Mark gets enough money to pay off his rent, and decides he can use his power not only to impress Anna, but also stop his neighbor, Frank (JONAH HILL), from having suicidal thoughts, and also help his ailing mother, Martha (FIONNULA FLANAGAN), die with the belief that there's an afterlife.
When others overhear that, Mark ends up becoming something of a prophet, making up a story about a "man in the sky" who controls everything, including choosing where one goes after death. From that point on, Mark must deal with the repercussions of his lies about that as well as his desire to win over Anna despite her honest if callous objections to his genetic makeup.