Mrs Warren, a London lodging-house keeper, comes to Holmes with a curious story of a reclusive and eccentric tenant. Mrs Warren's husband has been abducted and then quickly released again, perhaps a case of mistaken identity. Holmes decides the case is more important than it appears.
When Susan Cushing first consults Holmes about the disappearance of her sister Mary, he shows little interest in the case. But then, at Christmas, Susan receives an alarming present in a cardboard box, and Holmes is quickly on the trail of a dangerous criminal.