Very forgettable movie, so difficult to write up this one.
There is one scene-stealing moment by Stephen Merchant and Barry from Eastenders, who were always the best ones in the 'Extras' series, but it's over too quick.
A major flaw in the movie is that even though people can't tell a lie, which is fair enough, they insist on telling people every thought in their head. I'm not a brain surgeon, but that's not lying or even remotely linked to that whole concept. Also, early on during the meal, Gervais gives a "great" sarcastic remark about Anna's mum's welfare which would be classed as lying. Anyway, too easy, there'll be forums about all the mistakes so best cut straight to the ending!
His 'lie' about Heaven gets bigger and bigger, and under pressure he keeps digging the hole bigger rather than just lying to get out of it. Some lies are passed off easily as "well that's just your opinion" by Anna and Tina Fey, so it's not likely that the world would believe him. He might have just had a vivid dream - or be mad! Would they believe someone who genuinely thought that aliens had done experiments on them? Or the sky was green?
Anyway he becomes filthy rich and the most important person in the world - but all he wants is Anna. He writes a fictional screenplay to a 13th century event and wins awards as the best screenwriter of his generation, but he is still unhappy.
Given the chance to lie about genetics to Anna, he tells the truth (in his opinion, about short, fat kids) and she turns to Mark's rival Brad Kessler (Rob Lowe). On the day of their wedding, Mark shaves his beard and turns up with the help of his drunk friend.
He disrupts the service, wins Anna's affections, tells her the truth and they end up together because she decides she wants short, fat kids.
A few years into the future Mark and Anna are married with a young son - who has inherited his dad's ability to lie. But it seems they use their powers for good - to lie tactfully about the quality of Anna's cooking.