The theories mentioned above stating that current flowing through a resistance will result in heat, are of course all valid. This is exactly how your toaster works - mains current flowing through a length of resistance wire (normally in the form of a flat 'ribbon'). But this is specifically designed and manufactured to produce the desired heat output. But as stated by other iFixit boffins, you don't want this to happen in the outlet wall socket or the toaster's plug. The electrical connections here should be sound (tight) and corrosion/carbonated free and presenting close to zero resistance.
What very often happens is that only one plug terminal gets hot (not just warm but really hot). This terminal is usually the neutral (and surprisingly not the 'hot' terminal).
So, if this is indeed the case, and as already alluded to in previous posts, a solution is to 1. Open the plug, inspect the leads and wiring and test the tightness of lead securing screws. Fix any problems found here. I also then use a fine grit ememory paper to shine-up the plug terminals that connect to the wall socket. If too corroded/carboned-up, replace the plug.
If the plug is fine (or is a moulded one you can't open), then, as already suggested, check the wall socket for the same problems (screw tightness/corrision/carbon build-up). Clean, tighten and/or replace.
NB: ALWAYS FIRST SWITCH THE CIRCUIT BREAKER FEEDING THE WALL SOCKET OFF AT YOUR DB BOARD THEN TEST WITH YOUR MAINS FINGER-NEON INSULATED SCREW DRIVER (or similar mains detector/meter) BEFORE ATTEMPTING ANY WALL SOCKET INSPECTION/REPAIR/REPLACSMENT!!