There is more than one possible cause. One of which you can fix. If the plug prongs or the contacts in the receptacle get oxidized, usually caused by some burning or arcing or just plain aging, this will produce a point of electrical resistance. At this point power is consumed so heat results. It can be enough to melt the plug and wires!
The other possibility is that, at the point where the wire meets the plug, several of the internal strands of conductor wire are broken. Again, this causes a resistance point where heat is produced.
In the first case, you can just clean up the contacts. In the second case you must remove and replace the plug. Cut the wire back to a fresh point.