Bingham and Mary are getting married but Mary is applying for promotion and Lawrence fears she won't get it if it's known she's married, so Duncan tells him to keep the wedding secret and leave it to the last moment. The day arrives and Duncan, who is to be best man, and Lawrence, race through an operation, arousing the suspicions of Paul and Dick, who think Mary is marrying Duncan. The wedding goes ahead but the four doctors get into a scrap and fall into a fountain, which shows up on the wedding photos.

Waring and Bingham are on night duty in the emergency room, with sole patient Nobby Watts, when a sailor who has returned from Asia checks in - with all the symptoms of small-pox. Having rung the Medical Officer of Health to report the outbreak, the two doctors panic as they are bitten by the sailor's pet mouse and accidentally break the radiator, causing a flood. Next day it is revealed that the sailor had chicken pox, not small-pox, but the medics have to go into isolation since they were bitten by a - possibly rabid - mouse.