It looks as if someone plugged it into a different socket, proving
that the low resistance (bad connection) is in the plug. When taking
apart the plug observe the terminal condition. As mentioned
previously the wire to spade connection was probably compromised. Cut
off at least 4 inches to find bright copper, install a NEW plug maybe
soldering, or crimping on a spade, tighten securely, and plug it into
one of the unburnt sockets. Feel the appliance wire for abnormal
temperature. Observe any and all safety procedures.