Plantinga's argument defines God as a necessary being, and assumes it's possible that God exists. From this, and the S5 axioms of modal logic, it folllws that God exists. But you can just as well argue, "It's possible the Goldbach Conjecture is true, and mathematical truths are if true necessarily true, therefore the Goldbach Conjecture is true." Or even "Possibly it's a necessary truth that pigs fly, therefore pigs fly."

(This is as much as I can explain without trying to give a lesson in modal logic, which I'm not confident in my ability to do.)