In Days with My Stepsister, Yuuta Asamura's life takes a turn when his father remarries, bringing a new stepsister into his life, Saki Ayase. Both of them made a pact to maintain a certain distance between each other, not getting too close or too distant, as they've learned from their parents' past relationships.

Saki, who has shouldered responsibilities alone for her family, finds it challenging to rely on others, while Yuuta is uncertain about how to truly connect with her. Despite their initial reservations, they gradually find comfort in living together on more equal terms.

As the days pass, their relationship evolves from strangers to friends. This is a story between the duo that may one day lead to something even deeper — love.