Mazers. The Mazers, #5

Four people who have never met open four different doors on the same afternoon - one in a subway tunnel, one in a forest, one in a sewer, one in the ruins of a civilization that died ten thousand years ago. All four doors open into the same corridor. The corridor has no end, and it will not let any of them leave a world until they have done one genuine kindness in it. MAZERS strands them together - a boy with a machine buried under his city, a girl anchored to an alien, a cook who fights with a wok, and a cadet raised by a hive - in a place that was built, patiently and a long time ago, to teach.
Told in all four of their voices, one per chapter, across twenty illustrated chapters. Season 5 of the Isekai Nakama series.