Is what's reflected in the mirror a lie or the truth?

When the cherry blossoms fall.
The main cast of third-year middle schoolers hear a strange voice in the school compound one day.
When they follow the direction of the voice, they find a beautiful mirror.
The voice of a young boy saying, "Help me, help me".
As if inviting them in, the main cast is pulled into the mirror.
The sight they saw within was frightful enough to make them hold their breath.

It was the elementary school that should've been abandoned years ago...
Cowering in the place that shouldn't exist, they can't escape no matter what they do.

"I...committed a sin."

The owner of the voice they heard from the mirror was standing there.
Who is that boy?
Will they be able to escape?

The curtain rises on the stage dyed with fear and despair.

[Translated from the official website]