Northumberland, winter 1937. In a remote moorland cottage, Steven Coulter, a young history teacher, is filled with grief at the death of his wife. Through a charismatic colleague, Frank Embleton, and Frank's sister, Diana, he is drawn into the beguiling world of a group of musicians, and falls under their spell. But as war approaches a decision is made which calls all their lives shockingly into question. Moving between the isolated beauty of the moors, a hill-town school and a graceful old country house, Trio explores conscience and idealism, love and desire, and the power of music to disturb, uplift and affirm.