There are two tragedies in this film. The first is the death of Mary Jo Koepechne, the young woman left to die in an upturned car in shallow water. The second is Ted Kennedy's ultimate refusal to accept responsibility for his negligence. The fact that the second tragedy works so well in the shadow of the first is a testament to how well the movie is written and performed.
The movie begins with a montage of Kennedy family photos and audio describing the achievements and deaths of Ted's three brothers, Joe Jr., Jack, and Bobby. The implication is that Bobby lives in a world of tragedy already, but that he also lives in the shadows of three brothers. Throughout the film, we hear from Ted, his cousin Joey, the secretaries (including Mary Jo), and even Joe Sr. that Ted either needs to find a way out of those shadows or that he will never be able to get out from them. Ted wants to be his own man, but he can't quite escape the gravitational pull of his family name that, a mere decade before, had dominated American political life.
In seeming preparation for what could be a presidential run, Kennedy rounds up Jack's secretaries in order to try and get them to work for him. He invites them to a small cabin in Martha's Vineyard to drink and participate in a boat race (that Ted embarrassingly loses). The secretaries as a whole, save Mary Jo, have bought into the Kennedy mystique completely, but Mary Jo is scarred by the deaths of the Kennedy brothers and is reticent about coming back into the "family", as Ted puts it. Ted drives her back to town in an effort to continue to try and convince her, but he takes the ill-fated turn and drives off the bridge, overturning the car.
One of his first lines of dialogue after exiting the car cuts right to him. He's gotten out of the car (he doesn't know how) and walked back to the cabin. When he sees his cousin Joey out of sight of everyone else, he says, "I'll never be president." It's not the safety of Mary Jo whom he's left behind, probably dead, that concerns him, but his political future and his ability to live up to the family's expectations.
What follows is something between a comedy of errors and a tragedy as Ted, Joey, and advisors brought in by Joe Sr. work to mitigate the situation with Ted making poor decisions left and right from a sloppily written statement to the police (that gets to the media) to a neck brace that brings nothing but scorn due to its obvious needlessness. All through this, Joey, the faithful cousin lawyer, is Ted's conscience, begging for Ted to do the right thing by calling the police right after the accident, telling the truth, and eventually resigning.
It's the final scene, Ted's address to Massachusetts and the nation, that shows Ted as completely fallen and Joey's complete degradation in the face of Ted's insistence on weathering the storm. Joey has begged Ted to resign, even writing a resignation letter at Ted's request with the expectation that Ted would read it. But it becomes obvious that Ted's not going to do it. His voice seems distant. The lighting is low. There's something wrong, and he tosses Joey's letter aside in favor of the message written by Joe Sr.'s advisors. Joey's ultimate disgrace comes when he is shanghaied into holding up the cue cards for Ted in his final half of the speech. Joey does his duty, but he simply cannot believe what he's seeing and hearing from his friend. He had expected Ted to rise up above the shadow of the Kennedy family by doing what was right, but Ted had fallen in with Joe Sr.'s view of greatness instead.
It's that final scene that really makes the movie, distilling the central conflict within Ted by using Joey perfectly in contradistinction to Ted's actions.
