The story starts with telling of a legend about how one will be transported to another way if they look into the mirror in a enshrined in a shrine. And this story is supposingly fake as the elements in this legend are added over the years by the people, rather than it being passed down properly by the people.
Then it goes to the MC side with him, participating in a test of courage. And of course everyone knows what will happen next, the MC tried to show off to his childhood friend and attempted a selfie. Yes a selfie with the mirror! Gosh this is obvious that it is a flag for the activation of the supposingly fake legend. And the MC get transported to another world? that summoned him with a ritual.