Stained Glass ni Hikari o Tomoshite

The protagonist, Izumi Sumi Ao, wakes up in a hospital.
She has lost all memory of who she is and what happened.
Moreover, before her eyes floats a glass orb.
When she cautiously touches it, she finds herself not in the hospital, but in a church, where two girls are talking.
The words spoken by one of them, Yamakaze Midori, sear vividly into her mind:
"I'm sorry."
Why did she say that?
What has she forgotten?
Guided by the memories within the glass orb, she begins to reclaim her own past.