The Mazarin Stone, a famous jewel, acts as a magnet for crime. Holmes investigates the latest and perhaps most curious episode in its adventures.
Lovejoy takes Charlotte to an Antiques Fair at Cambridge, where the elderly Doncaster sisters, who have financial problems, invite him to value their belongings. Lovejoy finds what seems to be a very rare bible, but the sisters' brother, a Cambridge don, opposes its sale. Lovejoy then discovers the bible is a fake and suspects mischief.
Meanwhile, Beth's father wants her to leave Lovejoy Antiques for something better.