Occupying adjoining suites on the same floor of the Maddox Apartments, Hilda Brown and Henry Brown, strangers to each other, become acquainted and fall in love. In another part of the city, Hope Crewes and Charlie Hollis, sweethearts, have a quarrel and separate. Hope, who is a friend of Hilda's, writes she is coming on for a visit. The note also requests Hilda to leave her key with the janitor should she happen to go out. Charlie and Henry are also friends, and the former sends a similar letter to the latter. Hope arrives first and, Hilda having gone out, she is given the key to Henry's apartment through the blundering stupidity of the thick-headed janitor. Charlie arrives a little later and another mistake occurs when he is given the key to Hilda's apartment. Hope enters Henry's rooms and proceeds to make herself at home, while Charlie does likewise in Hilda's apartment. When Henry returns, he is puzzled and angry to hear someone in his bedroom, and is cogitating what to do when Hilda, who has discovered Charlie in her room, locks him in and rushes to Henry for help. She is highly shocked at seeing some ladies' wearing apparel in Henry's room, and when he is unable to satisfactorily explain matters, a severe quarrel is the result. Simultaneously, Charlie and Hope get out on the fire escape and come face to face. After more apologies and frantic explanations of each other's presence, the ludicrous affair is straightened out when all four chance to meet in the hall. When it is made clear how the error occurred, all roar with laughter, Hope and Charlie make up, as do Henry and Hilda, and soon they are laughing about how they got into the "Wrong Flat."
—Moving Picture World synopsis