By the end of his life, Senator Edward M Kennedy was “the lion of the Senate”, a sturdy marble column of American liberalism for close to 50 years. His final substantive act was giving the Obama daughters their pet dog. But in 1969, though he was a powerful man on paper and next in line for the presidency, family insiders knew he was a joke.

Joe Kennedy Jr, who died a war hero, was the favorite. John F Kennedy, the martyred president, had the charm. Robert Kennedy, who was supposed to be president, was the brilliant one. All this landed on the shoulders of Ted, the last living Kennedy son. He never had the respect of his father, but still lived in the glow reflected by admirers of his brothers, especially after the death of Bobby.

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This is all mapped out in Chappaquiddick, directed by John Curran and written by Taylor Allen and Andrew Logan. The screenplay is not adapted from any specific researched book, and when one of the themes is deception it’s hard to know just how much of this actually happened. Maybe that’s part of the point. One leaves Chappaquiddick, named after the island where the Kennedy associate Mary Jo Kopechne drowned at the bottom of a pond, awash in ambiguity. It’s never a question of whether Kennedy’s behavior was wrong, it’s: how much are we supposed to hate him? Put bluntly, if you had immediate access to the most powerful network of political fixers at your disposal, just what would you do?

Jason Clarke is strong as the weak senator, and he wisely goes easy on replicating the unmistakable Massachusetts accent. We meet him just as he’s to take off for the weekend, an important one for the family legacy, with Apollo 11 about to land on the moon. This is framed as John F Kennedy’s final action here on Earth and beyond. But it’s also a weekend in which a party with the “boiler room girls” (young women who worked on the Kennedy campaigns) is planned on Martha’s Vineyard.

Facebook Twitter Pinterest Edward Ted Kennedy after being charged over the Chappaquiddick incident Photograph: AP

Among them is Kopechne (Kate Mara), a brilliant and loyal young woman who the Kennedy cousin/confidant/lawyer Joe Gargan (Ed Helms) is hoping will return for Ted’s inevitable presidential bid in 1972. Ted and Kopechne pair off for a drive and the rest is history, even if we’ll never know what really happened.

Over the years, there have been plenty of conspiracy theories, from Ted being set up by Richard Nixon’s CIA to Ted intentionally murdering the young woman to keep her quiet. What probably happened is what we see in the film: an accident caused by booze and a panicked man hoping it would all go away.

The push and pull of this movie is really between the lead character and the audience. One moment we feel compassionate: he wants to do the right thing. The next moment he’s gone down the cover-up rabbit hole and is an irredeemable swine. John Curran has a light touch, though. Even when we most hate Ted, we don’t hate the film. Indeed, the cruelest scenes are when things really kick into gear, as when the handlers at the Kennedy Compound start talking about securing this poor young woman’s body.

The most repulsive moment comes when one of the Kennedy team schemers pulls in a favor from someone whom he knows to be “loyal to the cause”. One is reminded that the cause, and everything the Kennedys are supposed to stand for, is righteousness. Ted Kennedy could do the honorable thing and resign, but that’s certainly not what his wheelchair-bound father would want. (Bruce Dern plays Kennedy Sr, whose locked-in facial expression after his stroke is one of eternal disappointment.) One can debate if this is means the movie has a happy ending just as one can endlessly debate the morality of Ted Kennedy and power structures in general.