Workaholic assistant Atobe Arihito fell asleep in his company's bus that was traveling for a vacation. When he awoke he learned he had died due to the bus getting into an accident, and that he had reincarnated into a world of reincarnators - to the Labyrinth Country. To his surprise he was discovered by his slave driving manager, Igarashi Kyouka, who further explains their role in this world.
In Labyrinth Country, Reincarnators are expected to choose a job and form parties to fight against monsters in Labyrinths. "Valkyrie" Kyouka had expected Arihito to work with her once again, but he declines due to his knowledge of her demanding nature. As they depart she vows to make him regret not choosing her. And thus starts Arihito's adventure as a "Rear Guard".
Based off of the LN/WN series of the same name.