A retired Irish nurse named Philomena Lee harbors a deep-seated secret she has kept for fifty years: as a teenager, she became pregnant and was sent to a convent. In the convent, her baby was taken from her and given up for adoption in America, as was the case with many unwed mothers in 1950s Ireland. Philomena has spent her life wondering about the fate of her son, Anthony, whose memory she cherishes in silence, unable to overcome the Catholic Church's strict policies on confidentiality.

Decades later, Philomena's resolve to find her son is renewed on his fiftieth birthday. Through a twist of fate, she meets Martin Sixsmith, a former BBC journalist and recently dismissed government advisor, feeling adrift in his own career and life. Despite his initial reluctance and skepticism about human interest stories, Martin is drawn to Philomena's unshakable commitment to finding her son. The unlikely pair embark on a journey together, leading them to the United States. As they follow the trail of Anthony's life, an odd-couple relationship develops between the worldly cynic Martin and the gentle, optimistic Philomena, driven by her love and longing. Their search uncovers a deeply personal and emotional story, challenging both of them in unexpected ways.