Agnes, an alcoholic and drug-user who is struggling to overcome her self-destructive behaviour, returns from Toronto, Ontario, to her Cape Breton Island hometown of Sydney, Nova Scotia, because of the failing health of her mother Rose. Rose, an Irish-Canadian who is also an alcoholic, lies dying of cancer at a local hospital. Agnes stays at her childhood home with her older sister Theresa, a devout Catholic whose husband recently left her for a younger woman, and Louise, a middle sister who has retreated from the outside world. Waiting at their mother's deathbed, they are forced to face the resentments, trust issues, and scars of their past, particularly the sexual abuse they suffered at the hands of their father, as they make peace with one another and with their mother. The sisters bring their ailing mother home despite the mistrust they feel at Agnes' pledge to care for Rose, but Agnes cleans the house, acts responsibly, and even encourages Louise to play her guitar and socialize with a friend from church. When Theresa's husband Donnie is left by his girlfriend, Theresa feels compelled to comfort him and clean his house as she blames herself for his betrayal; he wanted children while she didn't, which she considers a sin. Agnes repeatedly drives out to a craft and gift shop in rural Marion Bridge, near Sydney, where she befriends a 16-year-old girl named Joanie who works at the shop. When Theresa finds out what Agnes has been doing, she angrily warns her sister not to tell Joanie about her relationship to their family, and she adamantly refuses to consider Agnes' suggestion that they talk to their father. Eventually Theresa relents about Joanie and accompanies Agnes to meet her. Joanie's adoptive mother Chrissy comes to visit them and asks that they wait until Joanie is an adult before telling her their secret. It is implied that Joanie is the product of the incestuous relationship between Agnes and her father. When Joanie visits the sisters and asks Agnes whether she is her mother, Agnes tells her that Chrissy is her real mother. Before she dies, Rose asks her daughters to forgive her for ignoring things she didn't want to see as she believed it was best for everyone. The sisters finally visit their father, who is suffering from dementia, and his wife. With Agnes' encouragement, Louise buys a new truck and the sisters drive out to Marion Bridge for a picnic with Joanie and Chrissy.
In The New York Times review, film criticStephen Holden praised Marion Bridge as "exquisitely acted" as well as "truthful and quietly compelling," adding that "it uncovers a complexity and depth of feeling rarely glimpsed in a family drama."
