People probably expected something very sciency, with special effects, with the big contagion that gets solved by the brilliant scientist, but it is nothing like that. It just describes, in a pretty slow pace, the aftermath of a global contagion that makes everybody amnesic. People that love each other forget the other exists at random moments, smart intellectuals fight all day to remember who they were by reading their own books (man, that should be another movie altogether!), rich people have built bunkers in which they educate their children who, unfortunately, reach adolescence and become stupid, and so on. There is no storyline, no explanation, no closure. Nothing too brutal, nothing too bloody, just people being people without being themselves.
It is not a great movie, it will not be remembered throughout the ages as a classic of direction or acting or storytelling, but it's not bad either. It's a "what if?" scenario played close to the end and well interpreted.
