Sanae can see the lifespan of others and spends hers making them happy.

Himesato City.
Sanae had just taken the college entrance exams,
fulfilling a promise to her dearly departed friend.
Staring out the bus window on her way home,
Sanae saw an old house.
Her heart raced suddenly, and she was compelled
to go there. Arriving in front of the house,
Sanae fell off the face of a cliff.

When she regained consciousness, she was in a bed.
The fabric smelled of mildew.

Everything changed when Sanae met Mikoto.

Their meeting would make her know life, death, and the world between---.

[From DLsite English]