A slick, taut, reality-based little cyber-thriller that may be a mite dated in technological terms but still fascinates because of it sheer luridness, “U Want Me 2 Kill Him?” is based on a 2003 case in Great Britain, reported by Judy Bachrach in a Vanity Fair article of 2005, in which a teen tried to stab a classmate to death. The question is: why? Spiffily directed by Andrew Douglas and featuring skilled performances by Jamie Blackley as the perpetrator and Toby Regbo as his victim, the movie builds slowly to a last reel that offers a twist that Agatha Christie might have enjoyed.

The story is told from the perspective of Mark (Blackley), a horny high school soccer player who barely escapes the wrath of an angry boyfriend when he’s caught cuddling with classmate Zoey (Amy Wren) in her bedroom. Fleeing on his bike and retreating to his own house, he takes to his computer and engages in dirty chatroom conversation with a girl named Rachel (Jaime Winstone). Her tale of being in the witness protection program and being threatened by an abusive boyfriend named Kevin (Mingus Johnston) might sound a bit exaggerated, but Mark accepts it without question, no doubt because he’s too busy pleasuring himself while she’s telling it.

The boy is surprised, however, when Rachel informs him that she’s the older sister of his classmate John (Regbo), a geeky chap who’s regularly bullied, not only because he’s a weakling but because it’s rumored that his stepfather has terrorist connections. Rachel asks Mark to befriend John and he complies, going so far as to give John’s chief tormentor a taste of his own medicine.

Before long Mark is completed besotted with Rachel though he’s never actually met her, and he and John have grown to be almost constant chums. But his bliss is undone when Kevin finds out about his regular sessions with Rachel and threatens them both. It’s shattered completely when he visits John, who’s been missing school, only to find that Rachel has died after a fall from the roof of her apartment building. Though the police are unable to charge him because he has an alibi, everyone strongly suspects that Kevin killed her.

That sets Mark off on a mission of revenge so obsessive that it ruins his performance at school and even tests his friendship with John. Things get yet tenser when Mark discovers he’s being stalked by a shadowy figure, presumably Kevin. And they go completely haywire when Mark is contacted by MI5 agent Janet (Liz White), who not only warns him to stay away from Kevin but informs him that John has come onto their radar because he’s colluding with his terrorist stepfather in planning a mass assault on the school. Hardboiled and intense, Janet suggests that Mark might want to take matters into his own hands to stop a tragedy from occurring; and the boy, shocked at the suggestion, asks the titular question.

The film goes on to explain everything, including the identity of Mark’s stalker, via a structurally complex narrative scheme devised by writer Mike Walden that includes post-stabbing flash-forwards as Mark awaits trial in addition to a straight chronological thread. Douglas presents it all efficiently, getting strong performances from his two leads and striving to overcome the usual banality of computer-screen sequences in techno-heavy pieces by having the characters actually speak the lines they’re typing onto their devices, so that one can see the reactions they have to the other’s messages. The method has its drawbacks—Mark is too often seen chewing on his fingernails, and the film does cheat a bit in showing the varied people he’s chatting with (a tactic that can be defended by pointing out that everything is being shown from Mark’s highly suggestible perspective). Otherwise, the film is a technically proficient piece of work, with cinematography by Tim Wooster that uses a subdued color palette that effectively adds a sinister feel to the images.

One might note that simply on the basis of its plot, “U Want Me 2 Kill Him?” might be a Lifetime movie in the vein of “Cyber Seduction” (check it out). But it’s cleverer than the usual cable fare, and better realized too. You might feel more than a little manipulated by the plot twists in the final reel, but a lot of people think that after they set down one of Christie’s books as well; that’s just how the genre works, and either you accept the premise and let it carry you along, or you don’t. You have to be willing to suspend disbelief and go along for the ride.

And in any event the filmmakers can take refuge in Ed Wood’s old axiom: It actually happened! Except in this case, it did.