Kon, a swordmaster and leader of the guild Insanity Blue, was betrayed by his comrades right after finally killing the strongest boss of the game — the Tyrant Dragon. Before claiming his reward, the one-time wish, he was stabbed by Maki, his oldest friend in the game.
Dying in that game means dying in real life.
Soon after dying, Kon found himself awake. But the body was not his level 1000 swordmaster, nor was it his real body in the real world. He discovered he had been reincarnated in the body of his alt character.
Kon swore revenge. They took his life, now it was his turn to take theirs.