Searching for the true killer of Count Drebber, Sherlock and John set a trap for the culprit using a wedding ring found at the murder scene as bait. After a struggle in which the ring is lost, Sherlock is able to deduce who the real killer is.

With 48 minutes remaining before the train reaches the next station, Sherlock declares that William and he can find the murderer in time. Sherlock relies on the evidence at the scene, while William pursues a psychological profile of the killer.