If the fuse in the plug is fine then if its not working at all then its going to be the internall fuse inside the microwave thats blown.
Not sure if you know much about microwaves but what you need to do when you remove the outer casing is discharge the high voltage capacitor before touching anything in there. As a microwave can hold upto 4500v even when unplugged so if you touch a wire without discharging it it can throw you across the room as the voltage is very high so the capacitor has to be discharged before touching anything in there.
So what you need to do on this is remove the outer casing by undoing the screws on the back.
Then discharge the high voltage capacitor at the bottom with discharge leads or a well insulated screwdriver to earth.
Then from there look on the circuit board and you will find the fuse. Replace this fuse and see how it goes from there.
Fingers crossed its just that but if after replacing the fuse it blows again then theres a fault in there. Now it can be a number of things that cause this. Either the magnatron, transformer, diode, high voltage capacitor or the door switchs. So it can be either of them thats casuing the fuse to blow so they will need to be checked out to see which is faulty.
But as to the fault you have here it will be the internal fuse at fault here. If you need anymore info at all on this then just get back to me.