A plug-in diffuser, for use with a substance to be thermally diffused, includes first and second electrical assemblies, each including a male terminal and a female terminal electrically connected to the male terminal. A thermal cutoff mechanism switches from a conductive state to a non-conductive state at a threshold temperature. The male and female terminals of at least one of the terminal assemblies is electrically connected through the thermal cutoff mechanism. A receptacle holds the substance, and a heating element is connected between the first and second terminal assemblies for heating the substance held in the receptacle when the diffuser is plugged into an electrical outlet via the male terminals. The female terminals cooperate to form an electrical socket into which an external device can be plugged.