The high temperature pipe plugger disclosed uses a backup plate for the sealing element on the plugging member that provides substantially complete support for the sealing element against the differential pressure imposed on the element by the hot fluid in the pipe. The backup plate may either deform the pipe to match its outside diameter or the pipe may deform the plate to match its inside diameter depending on the material from which the plate is made and the shape of the pipe. A method of isolating a leak in a superheated steam line is also disclosed. The method locates the pipe plugs far enough away from the bypass openings for the steam adjacent the plug, being removed from the flowing stream, to lose its superheat quickly and thereby reduce the temperature at which the sealing element of the plugging member must operate while the line is being repaired.