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Chappaquiddick is a small island on the coast of Massachusetts. On July 18, 1968, Senator Ted Kennedy held a cookout at a cottage for some close friends and associates. That night, he got behind the wheel with a passenger, Mary Jo Kopechne, and accidentally went off a bridge into tidal pool. The Senator escaped the submerged car, but Kopechne drowned. Kennedy didn’t report the accident to the police until the next morning, and Friday a week later he addressed the incident to the public in a nationally televised speech.

Chappaquiddick covers the two Fridays and the week in between.

At the film’s core is Jason Clarke’s performance as Kennedy. Clarke and director John Curran present Kennedy as neither a villain nor a tragic hero. Rather, he comes off as a boy at heart, profoundly in over his head even before the accident. For anyone who’s seen Clarke’s turn as a hard-bitten CIA torturer in Zero Dark Thirty, the contrast here is remarkable.

Astonishingly, only 36 at the time, Kennedy had already been in the Senate for six years. The script, penned by Taylor Allen and Andrew Logan, portrays Kennedy as well-meaning, but unwillingly swept along by the momentum of his family’s ambitions and the public legend of the Kennedy name. The film occurs several years after John F. Kennedy’s assassination, and barely more than a year after Bobby Kennedy’s death, and their ghosts hang heavy over the proceedings. As if fate wanted to rub it in, the moon landing — the crowning achievement of JFK’s space race — was two days after the accident. We do get to see Kennedy’s genuine political talent, particularly a gracious speech to the women at the cookout, who participated in Bobby’s campaign. But there’s also a moment during a television interview when Kennedy mentions living in his brother’s shadow. It’s clearly meant as a rhetorical flourish, but then the reporter actually asks, what’s that like? The frozen stare Clarke gives in response cuts right to the bone.

Kate Mara doesn’t get a lot of screen time as Kopechne, but she makes the most of it. The cutaways to her final gasping moments inside the vehicle are agonizing. Kennedy insisted there was no affair, and Chappaquiddick takes him at his word. But Clarke and Mara’s interactions also suggest the possibility was in the air. The film implies Kennedy dealt with his unhappiness and the weight of his obligations by acting out. Driving after a few too many drinks was a relatively venal example that nonetheless delivered catastrophic consequences. By all accounts, Kennedy also had an absolutely epic capacity for infidelity. But the film only glances at this subject: In the few scenes where Kennedy is together with his wife (Andria Blackman) she treats him with cold fury.

Another small but crucial role is Olivia Thirlby as Kopechne’s friend and another member of Bobby Kennedy’s crew. She embodies the paradoxes of believing in a man like Ted Kennedy as a force for good. When Ted finds the two women on the beach, both clad in bathing suits, the first thing Thirlby does is inquire about his wife. Later, upon hearing about the accident and Kopechne’s death, her first question is, how can we help the Senator? Then there’s Bruce Dern, who turns in a monstrously aristocratic performance as Kennedy Senior. Even with the old man bound to a wheelchair and barely able to talk, Ted still lives in terror of losing his father’s approval.

But after Clarke as Kennedy, Chappaquiddick’s key player is Ed Helms as Joe Gargan. Kennedy’s cousin and long-time friend, Gargan served as a political pointman and all-around fixer for the family. It’s Gargan who Kennedy first comes to after the accident, who gets the job of cleaning up the evidence of liquor at the party, and who coaches Kennedy through those first horrible hours. It’s also Gargan who Kennedy first lets down, by waiting until the next morning to inform the police. This sets the emotional tone for what comes after: Kennedy’s initial shock and erratic behavior after the accident is totally understandable. It’s only as the night drags on, and it becomes clear he isn’t going to honor his promise to Gargan, that frustration begins to set in.

Kennedy is clearly capable of decency, bravery, and integrity. But he is also surrounded by a family political machine that holds out the perpetual temptation of the easy way. Slowly, we watch as Kennedy surrenders to moral inertia, and his behavior becomes increasingly self-justifying and self-pitying. When Kennedy tries to contextualize the events by noting everyone has flaws, what’s disturbing isn’t that he’s wrong, it’s that he’s so clearly practicing for a speech. Helms excellently captures Gargan’s desperation for, and eventual disgust with, the path his cousin is headed down.

Kennedy handily won re-election to the Senate in 1970. He would go on to become one of the longest serving members in the chamber’s history, instrumental in a sweeping array of progressive reforms. But the filmmakers are not really concerned with whether Kennedy’s political accomplishments make up for his actions at Chappaquiddick. They’re interested in something much more intimate. When the possibility of resignation comes up, the implication isn’t one of high principle; anyone who made these mistakes shouldn’t be in office. Rather, resignation is what’s needed for this particular man to save himself. If he doesn’t, he’ll be surrendering to the machine forever.

Chappaquiddick’s script is economical, the film isn’t overly long, and Curran keeps the pacing up. He also gets a big assist from Garth Stevenson’s score, which manages to be both haunting and propulsive at once. This is an extraordinarily sad film, but not in a tragic or heartrending way: It’s just a remarkably clear-eyed examination of common, mundane human frailty.