Mr. Nobody is an old man looking back at his life. This is shown in confusing montage. The same (young) man keeps dying and does not. It appears that the (senile?) old man tells different versions of his life, versions how things would have been, would he have made other choices.
The film contains well done adolescent (failed) romance, scifi, pompous philosophy and, like I said, confusing montage. Do not expect an easy film, but an ununderstandable clutter memories. This is done with a more than descent atmosphere.
Indeed, a ‘difficult film’. Not a bad one either.