(Left to right) Jason Isaacs, Martha Plimpton, Reed Birney and Ann Dowd star as grieving parents in "Mass," now playing at AMC Madison 6.

“Mass” is not an easy movie to watch, but it is a highly rewarding one. Writer-director Fran Kranz’s film essentially takes four characters, puts them in a room, and lets them talk. After some awkward pleasantries, they edge closer and closer to the heart of the conversation, a white-hot center of grief and rage.

In a prologue, we see the employees at a small church getting ready for the meeting — fussing over how to position the chairs, how much food to have available, where to place a box of Kleenex. The obsessing over details builds tension in the audience; we sense they’re focusing on these things so they don’t have to focus on what’s really about to take place.

Finally, the four participants, two sets of parents, arrive, and we understand why everyone was so nervous. Jay and Gail (Jason Isaacs and Martha Plimpton) are the parents of a teenage boy who was killed in a school shooting several years ago. Richard and Linda (Reed Birney and Ann Dowd) are the parents of the boy who killed him, who also died.

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The meeting is an attempt at “healing,” but what becomes quickly clear is that nobody in the room knows what that healing might be, or if it is even possible. Kranz’s screenplay charts the hesitant path as the two couples fumble toward some kind of connection, pull away, and then try again.

The performances are brilliantly lacerating, and we feel completely present with these characters as they work through their grief, step by painful step. Jay has tried to channel his pain into activism, to make his son’s death “mean something” somehow. Gail practically vibrates with suppressed anger as she talks to the parents of her son’s killer, and Plimpton shows how her son’s memory can simultaneously bring her immense joy and agony.

On the other side of the table, Birney’s Richard is almost unnervingly composed, saying all the right things (“Thank you for your honesty”). And Dowd is phenomenal as the killer’s mother, wracked with sorrow at her son’s crimes while still trying to hold onto the few good memories she has of him. All these people are, in their own ways, trying to maintain being good parents to their lost sons, even as the effort consumes them.

Kranz, an actor turned filmmaker (he played the stoner in “The Cabin in the Woods”) knows just how to work with the performers, when to pull the camera in close and when to give them space. The single-room staging of “Mass” would suggest that it would work as a play, but it’s the intimacy that film can provide that makes it so powerful, making the audience a fifth person at the table in this little room.

The film doesn’t offer any easy answers or false hope, but it does suggest the possibility of moving forward, that these people can at least live with their grief and guilt. “Mass” is an understated powerhouse of a movie that never takes a false step.

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