Night Moves” isn’t so much a thriller as thriller-ish. That’s not to say that the movie — which centers on a plot by a group of environmental activists to blow up a hydroelectric dam — disappoints. It’s a richly engrossing drama, so long as you understand that it’s aiming for the head, not the gut.

That means that the story builds up a powerful, almost unbearable level of suspense, only to refuse to release it in the end. Although that will surely let some viewers down, it will reward those who don’t mind leaving the theater with something other than tidy closure. This isn’t a movie where the good are rewarded and the bad are punished, but one that invites us to question what those labels even mean.

Much of the film’s first half is devoted to preparations for the bombing, which has been organized by Josh (Jesse Eisenberg), a humorless idealogue who lives and works on an Oregon farm providing produce to members of an urban agriculture co-op. Josh is almost sociopathic in his self-righteous fury about the salmon that have been killed by the dam just so, as he puts it to no one in particular, “you can run your iPod every second of your life.” Dena (Dakota Fanning), a dilettantish spa worker, is bankrolling the operation. Former Marine and ex-con Harmon (Peter Sarsgaard) — who seems in it as much for the explosion as for the message it’s intended to deliver — handles logistics.

It’s a group of odd bedfellows, and paranoia runs high, with no one really trusting anyone else.

As with her 2008 film “Wendy and Lucy,” director Kelly Reichardt holds a magnifying glass up to the threads of our fraying social fabric. For long stretches, we watch various members of the trio carrying out banal pre-bomb work, as if they were planning a potluck. Josh and Dena negotiate the purchase of a boat; Dena is sent to a farm-supply store to pick up 500 pounds of ammonium nitrate fertilizer that will fuel the blast; all three mix and pack the explosives, and then load them into the boat’s hull, which Josh has gutted of its furnishings. This kind of minutiae is hardly cinematic, yet it contributes to a mounting sense of dread.

The tedium is alleviated by occasional moments of high tension. When Dena is almost stymied in her shopping trip — she has to lie to persuade the farm-supply manager to sell her that much of a controlled substance without a Social Security card — it’s hard not to pull for her, even though we’re sickened by the thought of what she wants to do with it.

Reichardt’s neatest trick is getting us to care about three people who are about to do this dangerous, illegal — though ostensibly high-minded — thing. And then when it’s done, and the real-world consequences of their actions hit home, we feel the same sinking feeling — even betrayal — that they do.

“Night Moves” is really about the different ways in which the members of the group cope with those consequences. Sure, there’s some question of whether they’ll get away with it, and that propels the movie. But after a certain point, that doesn’t even matter. Responsibility, disillusionment and the corruption of principle are the film’s true themes, and it tugs at them relentlessly, pulling them up by their dirty, tangled roots. It’s a movie about environmental extremism, but it could just as easily be set in Congress or the Occupy movement.

Written by Reichardt and her longtime writing partner, Jonathan Raymond, “Night Moves” is not a chatty film. Arguments one way or another are few and terse.

When news of the dam bombing comes out, Josh’s boss succinctly dismisses it as meaningless “theater” that will change nothing. There are 10 other dams on the river that would have to be blown up to make a difference to salmon ecology, he says.

Whether he, or anyone else in the film for that matter, is right or wrong isn’t the point. “Night Moves” is about the difficulty of coming to terms with what lies downstream — just around the river bend and out of sight — from every little decision we make.

★ ★ ★ ★

R. At the West End Cinema and Angelika Film Center Mosaic. Contains violence, obscenity and brief nudity. 113 minutes.