About two years ago, a pharmaceutical company developed an "emotion-stabilizing drug" expected to be effective against mental symptoms (depression, anxiety disorders, PTSD, etc.), which became a major topic worldwide. All of humanity was forced to take this drug, but it was a failure. Due to side effects, it became difficult for humans to control their emotions, and at the same time, numbers began to appear in visible locations on their faces. These numbers came to be known as "Erosion Values." At first, they were just numbers, but one day, a person appeared whose Erosion Value became "∞". That person lost their sense of self and murdered their entire family on the spot. They were shot dead by the military as a dangerous individual. The next day, a person with an Erosion Value of "0" was confirmed. They died in their sleep of unknown causes (cause of death unknown). From then on, many people's Erosion Values hit 0 or ∞, and the world collapsed rapidly. Two years later, social systems have completely disintegrated: the government has vanished, police functions have ceased, hospitals and schools are closed, and TV stations have stopped broadcasting. The cause of Erosion Values hitting 0 is not clearly understood. Research was conducted for a while, but it was suspended when the world collapsed. As long as one's Erosion Value doesn't hit 0 or ∞, one doesn't encounter someone with an ∞ value, and food doesn't run out, one can survive. Childhood friends Guest and Yunagi have just reunited in this world.