Kirishima was playing the VRMMO Break Force Online (BFO) is murdered while connected to the game and the next thing he knows when he wakes up is that he has become Theodore (same name as his character in-game) that has been pushed into the river by his half brother.
He seems to have been transported into the game, somehow retains some of his skills as a mage from it and both lifetime's memories, so he decides to leave the family that doesn't appreciate him and to travel and have adventures. His first stop, Tamwils, a labyrinth city, to find out if he's knowledge of the game applies in this new world.

(Source MU)