Boku no Daisuki na Oba-san

Yuuta Kusunoki's neighbor is Ikumi Shinoda, a woman much older than him.
Yuuta has always admired Ikumi and she has been like a mother to him.
One day, Yuuta goes to visit Ikumi to play and finds her sleeping on her couch.
A teenager like Yuuta understands "adult things" to a degree and soon finds himself "laying" his eyes on her.

When Ikumi wakes up, Yuuta quickly tidies himself and act as if nothing happened.
Embarassed, Ikumi gets up and sees that Yuuta is holding a piece of crumbled tissue in his hand.
Remembering that he forgot to throw the tissue away, Yuuta quickly comes up with a random excuse.
With Yuuta's flustered attitude, Ikumi sees through his lie asks if he is hiding something.

Ikumi then tells Yuuta that,
"It can't be helped if you can't tell me, but if you did something bad, it would be great if you do."

Guilt bottling up in his heart, Yuuta confesses what he did.
Although Ikumi ultimately forgives Yuuta, by a slip of tongue, Yuuta reveals his feelings towards her.
Taken aback by all this, Ikumi took it upon herself and blamed herself for not noticing.
She thinks that she is to blame for suppressing the feelings that Yuuta has.
She firmly decides to fulfill what Yuuta wants all this time by acting as his lover for just one day.

At least that is what the original plan was anyway...