With the third floor behind them, Kirito and Asuna reach a fourth floor the game has rebuilt as a water world, where chalk-white streets and gondolas have replaced the dry valley the beta testers remember. Securing a boat of their own brings them closer to the strange quest waiting there, until a faction of Fallen Elves crosses their path and turns the survey into a fight neither is prepared for. This manga adapts the fourth-floor volume of the Sword Art Online: Progressive light novels.