The invention comprises a latching and unlatching device for use on a door, or bulkhead, or the like, which device has a blocking plate for internally obstructing the latch to prevent its retraction, thereby to prevent the opening of the door. The blocking plate is internally secured in place, in an intermediate slidable position, where it does not obstruct the latch, by a low melting point cover. In the presence of excessive heat, as on the occurence of fire, the cover melts; this allows the blocking plate to slide to another, operative position in which it prevents the retraction of the latch and, consequently, the unlatching of the device. The method comprises the steps to be undertaken to modify an off-the-shelf latching and unlatching device to insure that it will be held securely latched in the presence of fire or excessive heat, and the kit sets forth the structure elements required for the aforesaid modification of off-the-shelf, latching and unlatching devices.