My experience at Hot Docs kicked off in a big way with The Imposter. Even though it was the first film that I would see at the festival, I can say now that it is probably going to be the one to beat.
The film is done in a very similar style to the films of Erroll Morris. That is, the film makes heavy use of re-enactments to recreate events as told by the various talking head interviews.
The imposter of the title is among one of the many interviews and he describes how he was able to convince a family that he was their missing child. That's pretty much all I will say about this particular individual. Though I will say, that your jaw will likely drop when his true identity is revealed.
A stand-out character in this documentary (and he is indeed a character) is a private investigator named Charlie Parker. This old man, who was the first to really suspect that something was up, is very much an eccentric character and the film was an absolutely joy whenever he was one screen. In fact, would it be too much to ask if they make a documentary about Parker and his investigations?
Overall, I would say that The Imposter was one of those "so odd it had to be true" stories and it made a very entertaining documentary.