This is one of the most insane, nerve racking movies I've ever seen. It's got an ingenius concept. You wonder why no one thought of it before:
An architect breaks up with his girlfriend and she moves out. A guy comes by and asks to use his telephone, they still have those in Spain I guess and he's like fine and leaves the guy alone to make the call. He doesn't see the guy leave and from the point on is convinced the guy is in the house and it takes over his life. He finds clues that the guy is still there. He borrows a neighbors dog to try and sniff him out, he calls the police over. Is he paranoid or is the guy really there?
The movie takes twists and turns I did not at all expect. Some people may find it too weird or just completley unfathomable that a guy would put himself through this but I didn't care and was utterly riveted. it does get a shade ridiculous in places and I didn't quite understand the ending. still