BAD SAMARITAN -- and not such a great movie, either

I wish I could tell you that “Bad Samaritan” turns out better than it does.

The movie certainly had potential -- especially with its unique plot:

Two young men use their valet-parking business as a cover for burglary; while the drivers are dining, they punch in GPS and drive to the owners’ homes, pilfering valuables while the houses are empty. On a certain night, one of them enters an up-scale domicile only to find a young abductee tied up and gagged; but she’s bound so skillfully that he can’t get her free. Other discoveries soon suggest that the wealthy kidnapper is also a psychopathic torturer.

So what should this “bad samaritan” do? Call the police?

For about an hour, screenwriter Brandon Boyce rings some cool changes on this clever scenario; but in its final 40 minutes, the movie flies right off the rails, throwing believability out the window while dragging the characters through a brutal, bloody climax that goes on far too long.

Robert Sheehan enlists our sympathy as the titular protagonist who wants to help the victim; he even manages to fill in some plot-holes by convincing us that he was too panicky to think of common-sense solutions like giving this woman the home-owner’s phone and then leaving.

Simple omissions like this keep piling up, however, and the last 20 minutes are laughably preposterous. Too many likable people die or get hurt, while too many others recover quite suddenly from grave injuries. Having sat through a reasonably smart and twisty first half, it’s terribly disappointing to watch this thriller descend into a run-of-mill chase in the woods, where the characters alternately bash each other with a shovel and a stick.

For me, the movie’s principal interest was David Tennant, who plays the abductor. The Scottish actor has some notable films on his resume (“Ferdinand,” “Harry Potter & the Goblet of Fire”) and is perhaps best known for TV’s “Broadchurch” and “Doctor Who” (he was the 10th lead in that long-running series). I especially admire his performance as Hamlet in the 2009 BBC production (which is, incidentally, available on YouTube in its entirety). So it was fun to watch Tennant flex his muscles as a skillful psychopath in the Hannibal Lecter tradition. He’s sufficiently menacing, but Boyce doesn’t give him enough to do. As is often the case, this villain is a bit too slick, and a few too many steps ahead of everyone else -- while the ultimate explanation of his motive feels trite and uninspired.

Jacqueline Byers provides excellent support as the valet-burglar’s girlfriend, and the film’s editing and photography are likewise top notch -- which makes its pedestrian ending all the more of a let-down.

In the biblical parable for which this film is named, two religious leaders ignore an injured man at the side of the road. While Jesus actually indicts them for passing by, it might have been wise for Tennant to do the same on this particular script.

He ought to be above this sort of thing.

“Bad Samaritan,”

directed by Dean Devlin

Run time: 110 min.

* * (out of four)

Rated R for violence, language throughout, some drug use and brief nudity