A silly thriller can still afford some fun, a fact proven by “Bad Samaritan,” which, while ludicrous in the extreme and overextended, boasts an over-the-top performance by David Tennant that almost makes it worth seeing. No wonder it comes from Dean Devlin, whose track record runs to the ridiculous, which he has sometimes elevated to near-camp status. In this case, working from a script by Brandon Boyce, he comes up with something that’s akin to an even more outrageous version of “Disturbia,” in which Shia LaBeouf went up against the serial killer next door.

Of course, Sean Falco (Robert Sheehan), an Irish expatriate living in Portland, doesn’t have a place next to the modernist mansion of ultra-rich Cale Erendreich (Tennant). He’s just a poor schlub, an aspiring photographer who gets by as a valet parker at an Italian restaurant, where he partners with good buddy Derek Sandoval (Carlito Olivero). The two have a nice scam going: when a customer drops off a car, one of them drives it to the owner’s home during the meal and robs it of a few items, preferably some that won’t be quickly noticed. Sean prefers to hit only the snooty patrons, but Derek is less picky, as we see in an early, unnecessary episode.

When sneering Cale shows up in his Maserati, Sean is happy to take off to his place, and joyfully pockets a newly-arrived credit card, which he registers from the guy’s home phone. But continuing to search the place, he comes upon a young woman, Katie (Kerry Condon) painfully shackled to a chair in Erendreich’s locked office. He tries to free her, especially after discovering a torture chamber in the garage, but has to leave with a bolt cutter from the monster’s tool collection when Cale returns sooner than expected.

The incident turns Sean away from petty crime: determined to save the girl, he calls the cops, only to watch as Erendreich cons them into leaving. Wracked with guilt, he goes to the police again, and when that fails, to the FBI. But Cale proves remarkably adept at clearing his place quickly of all incriminating evidence—where he stashes all his torture chamber devices is never made clear, but they vanish into thin air when a detective shows up to investigate the place (he’s played by an actor named Tony Doupe, which seems oddly descriptive), and though the federal agents are more interested in his story, they’re pretty slow on the uptake, too.

Even worse, Cale has identified Sean as his enemy, and takes aim not only at him, but at his relatives and friends, including his girlfriend Riley (Jacqueline Byers) and Derek. And when Sean goes back to Erendreich’s house in search of proof, he finds the evil genius has rigged the technologically cutting-edge place to finish him off; Cale is literally willing to blow everything to smithereens in his made search for revenge.

Our young thief-turned-hero nevertheless survives to track Cale to a rural cabin where the madman is holding Katie. In a ridiculously protracted final showdown, Tennant gets to yammer on endlessly about what a genius he is; we also get filled in, to some extent at least, on the killer’s motivation, which has been telegraphed in a prologue—a barrage of psychobabble involving a long-ago murder and his obsession as a boy (played by Austin Leo) with training show horses. But good eventually triumphs, and Cale gets his just deserts in a tableau that invites comparison to Hannibal Lecter.

All of this is frankly goofier than the most outlandish episode of “Criminal Minds,” and yet you have to have grudging admiration for the conviction with which everyone plays the foolishness. Sheehan positively oozes Irish (Catholic?) guilt as the young man returning from the dark side, and Olivero, Byers and Condon play along gamely. Devlin and cinematographer David Connell work hard to fashion a brooding atmosphere (along with a catalogue of “gotcha” moments), abetted by Joseph LoDuca’s score, although Brian Gonosey’s editing could be sharper, especially in that ponderous finale.

But what you’ll come out of the movie remembering is Tennant’s go-for-broke turn, which is at the other end of the spectrum from the tortured cop routine he worked in the superb English series “Broadchurch” (and its inferior American remake, “Gracepoint”), let alone “Dr. Who.” His scenery-chewing knows no bounds—and to be honest, there’s a unrelieved quality to the performance—but it certainly juices up a piece that fails to achieve the inspired lunacy of a piece of really great pulp trash like “The Stepfather” (the original, of course, not the remake), but can provide some modest pleasure if you’re in the right mood.

The original title of “Bad Samaritan,” incidentally, was “No Good Deed.” That makes a lot more sense, but one can sympathize with those who decided to change it, since it would have inevitably invited a host of easy critical barbs.