Thomasina Elliott never expected to keep in touch with lonely orphan Anna Ball after they left school. However when Anna wrote to her, she felt obliged to reply. The correspondence between them was constant for years until Thomasina found herself waiting for a letter that never came.

Thomasina has no particular reason to believe that anything has happened to her old friend, but when she makes some enquiries she discovers Anna has disappeared without a trace. Deeply concerned, Thomasina turns to Detective Inspector Frank Abbott, who asks Miss Silver to investigate.