Five hundred years after monstrous Beasts wiped out humanity, the last surviving races cling to life on floating sky-islands, defended by young Leprechaun soldiers who wield ancient relic weapons. Willem Kmetsch, the final living human, wakes from centuries of suspended animation and can no longer fight, so he takes a caretaker role at the Leprechauns' warehouse instead. Among them is Chtholly Nota Seniorious, wielder of the sword Seniorious, who warms to him despite having been trained to treat her own death as routine. This manga adapts the original light novel's post-apocalyptic setup.