In 1846, Herncastle visits his sister to wish his niece, Rachel, a happy birthday. His sister refuses to see him, and he vows that he will always remember his niece's birthday- a vow which later takes on sinister undertones. Two years later, just before Rachel's birthday, a man named Franklin Blake announces to Rachel that the Moonstone has been bequeathed to her by Herncastle.

Blake gives her the jewel on her birthday and offers to mount the jewel for her, in order that she might wear it. Inevitably, the jewel is found missing the next morning and Rachel believes Blake stole it. Determined to prove his innocence, Blake leaves in order to pursue the real truth behind the theft. Starring Greg Wise, Keeley Hawes, Terrence Hardiman, Mellan Mitchell, Peter Vaughan.