One of the strangest things in this documentary is that one of his former girlfriends looks almost exactly like Mary Kay.
As to the movie, this is the story of Timothy Treadwell, not his real name, who tried a lot of jobs and then decided to go to a forbidden area of Alaska to save the bears. He talks to the bears like pets -- Oh, so pretty, such a good baby, I love you -- in baby voice and gets close to them. In the end, a bad bear kills him and the girlfriend with him (only a couple of pictures and he never mentions her). We see his parents, friends, ecologists, etc., all telling us about their part of what happened.
Everything I see written about it makes him sound brave and self-effacing, but I think he was mentally ill. He doesn't actually watch bears from a distance, he tries to make friends with them, and that's nuts.