This takes place in an alternate world where people are incapable of lying - this not only means social niceties are largely absent, there is no fiction, since that is just consensual lying. In this universe works Mark (Gervais), a screenwriter, who does "lecture films," in which people recite historical events; he's burdened with the 13th century, a very boring era. He tries, unsuccessfully, to start a relationship with Anna (Garner), who is attractive, rich and successful - everything Mark is not. He is fired and about to be evicted, when he discovers something incredible: he is the only person in the world who can lie. Initially, this is a fabulous, and he uses it to turn his life around (though not without road-bumps on that journey). However, he ends up making up a story of the afterlife to comfort his dying mother: in other words, he has basically invented religion and become its messiah, since the nurses who overhear the story have no reason to believe it isn't the absolute truth, and waste no time in spreading the new gospel. Mark has to continue the deception or admit his new-found talent. As Voltaire once said, "If God did not exist, it would be necessary to invent him."
It's a remarkably subversive concept, and probably the most unashamedly atheist movie to come out of Hollywood in a very long time - an angle downplayed significantly in the trailer. The religious aspects provide the best moments, with Mark writing down the rules on a pair of pizza boxes, and proclaiming them to the assembled masses, only for them to start picking holes in the hastily-constructed fabric of his theology. However, it doesn't have quite the full-blown courage to go for the satirical jugular in the way, say, Idiocracy did, softening its message with the love story between Mark and Anna, as he tries to save her from making a huge mistake and marrying Brad (Lowe). I'd loved to have seen more of the world which results from the central conceit: it's built not just on an absence of deceit, but brutal honesty. This results in things like Coka advertising itself with "Because we're famous," and Pepsi as being, "For when they don't have Coke." However, there is easily enough humour to propel this forward, and it's a pleasant surprise to see a Hollywoood comedy with a brain in its head. Those of strong religious convictions might do well to steer clear, however.