The film follows the life of Mark Bellison, played by Ricky Gervais. Mark is an unsuccessful writer who lives in a world where everyone tells the truth, no matter how hurtful or embarrassing it may be. This includes everything, from bluntly stating someone looks ugly to revealing intimate personal details.
Mark is in love with his co-worker, Anna McDoogles, played by Jennifer Garner. When Anna tells him she cannot continue seeing him because he is not genetically desirable, Mark becomes depressed. He goes to the bank to withdraw his last remaining pennies and, to his surprise, the teller, played by Tina Fey, believes he has more money than he actually does. On a whim, Mark tells her that he has more money than he does, and she believes him.
Mark realizes that he has invented lying and uses it to his advantage. He tells his dying mother beautiful stories about heaven to comfort her, and people believe in his lies. He becomes rich and famous by fabricating stories about a man in the sky who decides everything in the world.
As Mark's lies become more elaborate, they start to spiral out of control. The people he loves start to question his stories, and the world he has created begins to crumble. In the end, Mark realizes the harm he has caused by inventing lying, and he decides to take responsibility for his actions.
The film is a comedy that explores the philosophical implications of a world without deception. It raises ethical and moral questions about honesty, trust, and human nature, while also being a light-hearted and entertaining story about one man's journey towards success.