César (Luis Tosar) has a problem: he has never been happy in his whole life. No matter what happens to him, he always sees the downside, and this constant misery is bringing him to the brink of self-destruction. But he has had someone in his life to keep him going, and she is Clara (Marta Etura), who lives in the same apartment building as he does. That is because she is a tenant and he is the concierge, who has a small room in the basement all the better to be around to see to the residents' needs. But there are some needs Clara would be better taking care of herself. Not that César would let that happen.
Spanish director Jaume Balagueró rose to prominence on the international horror movie scene with REC, a zombie epic constrained by the walls of one apartment building, and as if recognising the strength of what a potentially restricted and claustrophobic setting such as that could be, he returned to it with Alberto Marini's script for Sleep Tight, or Mientras duermes as it was called in its native land. This was not an all-out gorefest however, though there was one scene which was very bloody indeed, but an thriller with horror inflections as insidiously sinister as its main character. César is not a man to be pitied, you see, as he is conducting some very unpleasant activities.
And all so he can exert something he doesn't have much of otherwise: control over other people. As he is at the beck and call of the tenants in the block, he grows resentful, and starts to secretly break into Clara's home to spy on her, then take his surveillance to seriously queasy extremes. What made this particularly effective at getting under the skin of the viewer was the manner in which the film acknowledged that if someone is dead set on ruining your life, it is they who hold the power over you rather than the other way around. They may be obsessed with you, but they know how to pull your strings and as this gives them so much perverted pleasure it's going to take a lot to get them to stop.
If they're clever, as César is, they can evade the police even if the victim calls them in, and it was true that the villain here applied such skill to his machinations that he may be too difficult to believe as a convincing character for some, but Balagueró took care to ensure his profile as an offender was revealed by degrees so that by the time you really should be questioning the whole situation you're too deep into the plot. What César likes to do is hide under Clara's bed, wait till she's asleep, then emerge and drug her so that she won't wake up. Then, as far as we can tell, he goes to sleep beside her, setting his wristwatch alarm so that he's up before she is and suspects nothing.
But of course, he's up to more than that, and the longer he gets away with his crimes the further they intensify. Soon he is injecting an irritant into Clara's makeup to give her a skin condition, has spread fly and cockroach eggs around the apartment so that she has to call him to fumigate the place, and as if that were not menacing enough, he has the gall to continually send her messages (texts, letters, e-mails) goading her about his watching of her, knowing full well she cannot do a thing about it as he has covered his tracks too well. Clara is so sympathetically played by Etura, working well in what could have been a snivelling victim role in the wrong hands, that the fact her tormentor is succeeding make Sleep Tight a very uncomfortable experience. There are moves towards César receiving his comeuppance - his boss warns him about arriving late when we know what he's really up to, the little girl of one apartment knows exactly what he's doing and blackmails him - which makes for an ending very few will like. Music by Lucas Vidal.