Hunkeler's secret

Hunkeler, now a retired inspector of the Basel police force, is hospitalized and sharing a room with Stephan Fankhauser, an old acquaintance terminally ill with cancer. One night, a groggy Hunkeler wakes up to see a young nurse with a ruby ring on her hand administering an injection to his friend. The following day Fankhauser is found dead. Was the injection just a dream? Does the night nurse not usually wear a small diamond ring? There was no autopsy and a quick cremation. Hunkeler resolves to get to the bottom of the matter despite the objections of his ex-colleagues, who want the retired inspector to stay well clear of the investigation.