Major Petkoff's coat makes a significant contribution to the development of the plot. Much of the action of the play hinges on Bluntschli's 'borrowing' of Major Petkoffs coat and its return, which builds the play's suspense.
Bluntschli, a Swiss mercenary fighting on the side of the Serbs, in a bid to escape from his pursuers, strays into the bedroom of Raina, Major Petkoff's daughter. He needs to escape and Raina, already attracted to him, lends him her father's coat to facilitate his escape. The coat is therefore used to advance the plot as it serves as a vehicle for Bluntschli's escape from Bulgaria. Raina places her portrait in the pocket of the coat. This raises further complications in the plot of the play. When Bluntschli returns the coat, he gives it to Catherine with Raina's portrait still in the pocket. Major Petkoff has noticed that the coat is missing although his wife has tried to convince him to believe the contrary. When he finds the coat, he sees Raina's portrait in it.
He, however, keeps quiet, waiting for a satisfactory explanation. As he is preparing to go with Major Sergius and Captain Bluntschli for the rehabilitation of the Bulgarian soldiers, he calls for the coat. Cleverly taking the coat from Nicola, Raina stealthily removes the portrait before putting the coat on her father. To her father's surprise the portrait is no longer in the pocket. This development provokes an anxious moment of accusation about who may have removed it. To everybody's surprise but relief also, Bluntschli speaks up and the whole story surrounding the escape is told at no cost to the major players. The matter is, therefore, resolved to the satisfaction of all.