Christmas 1954. Wealthy philanthropist Rachel Argyll is murdered at her family estate Sunny Point. Her adopted son Jack Argyll is arrested for her murder. He vehemently protests his innocence.

Eighteen months later, Dr Arthur Calgary (Luke Treadaway), a mysterious scientist, walks onto the velvety lawns of Sunny Point claiming to have just returned from an expedition to the Arctic. Even more extraordinary is his claim to hold the alibi that can prove Jack's innocence. But Jack died in prison before the case could come to trial and the Argyll family is reluctant to dig up the secrets of the past.

Rachel's widower Leo (Bill Nighy) is about to marry his secretary Gwenda (Alice Eve) and non of Rachel's other adopted children Mary (Eleanor Tomlinson), Mickey (Christian Cooke), Tina (Crystal Clarke) or Hester (Ella Purnell), nor longstanding housekeeper Kirsten (Morven Christie), is willing to reopen that most horrendous chapter of their lives. However, the shattering implications of Calgary's story are too big to avoid; if he is telling the truth then the wrong person was arrested for Rachel's murder. And if Jack is innocent, then it must have been somebody else at Sunny Point. Somebody still out there...