London journalist Martin Sixsmith finds himself out of work after losing his position as a government adviser. At a social gathering, he is approached by a woman named Philomena Lee, who shares a heart-wrenching story about her mother. Philomena encourages Martin to write about her mother’s past, which involves giving up her son, Anthony, nearly fifty years ago. Initially hesitant to delve into a human interest piece, Martin meets Philomena and is moved to investigate her tragic story.
Back in 1951, Philomena, under her father's pressure, was sent to Sean Ross Abbey in Roscrea, Ireland, to give birth. After her son was born, she was compelled to work in the convent's laundry for the next four years and had minimal contact with her child. The nuns, without giving Philomena a chance to say goodbye, arranged for her son to be adopted, hiding this painful secret from her family for decades.
Determined, Martin and Philomena start their investigation at the convent. The nuns misleadingly inform them that adoption records were destroyed in a fire, yet they conveniently retain the contract Philomena had to sign, which forbade her from ever reaching out to her son—a detail that raises Martin's suspicions. At a local pub, they hear claims suggesting that the convent deliberately destroyed the records, exploiting vulnerable mothers like Philomena by selling their children for £1,000 each to wealthy families in America.
When their inquiry in Ireland yields no results, a hopeful lead from the United States prompts Martin to invite Philomena along for the journey. With the help of his contacts, they uncover that Anthony had been renamed Michael A. Hess and rose to prominence as a lawyer and senior official during the Reagan and George H. W. Bush administrations. A shocking twist emerges when Philomena spots Martin in a photo with Michael, recalling a past encounter. Tragically, they find out that Michael has been deceased for eight years.
Eager for closure, Philomena wishes to connect with those who knew Michael. They visit a former colleague who reveals Michael’s sexual orientation and that he succumbed to AIDS. In their quest, they also meet his sister, Mary, who shares heart-wrenching details about the abuse both siblings endured from their adoptive parents, along with stories of Michael's partner, Pete Olsson.
After initial reluctance, Pete agrees to discuss Michael's life with Philomena. Surprised, they view videos of Michael's final days, where he expresses his return to the Abbey in search of his birth mother. Pete discloses that the nuns had deceived Michael into believing he was abandoned by Philomena, which added to the tragedy of their untold story. He reveals that, contrary to his family's beliefs, he had Michael laid to rest in the convent’s cemetery against their wishes.
Determined to find Michael's grave, Philomena and Martin return to the convent. In a moment of anger, Martin confronts an elderly nun, Sister Hildegarde McNulty, who had denied Michael the chance to meet his mother. Their heated exchange unveils the hypocrisy of her actions, as Martin challenges her unrepentant stance, asserting that her behavior lacks any Christian compassion. In an unexpected display of grace, Philomena chooses to forgive Sister Hildegarde, revealing her remarkable strength. Standing at her son's grave, Martin shares his decision not to publish the story they’ve uncovered, but Philomena urges him to do so anyway, emphasizing the importance of their journey for truth.
