Kimika had two sons, but twelve years ago her divorce was finalized, leaving her eldest son in her care while her younger son, Yuki, was taken into her ex-husband's household. Eventually, the eldest son became independent and moved out, and Yuki began living with Kimika again to make commuting to university more convenient. Unlike his brother, Yuki was fiercely independent and disliked relying on his mother, which only deepened Kimika's affection for him. Wanting to strengthen their bond, she decided to take him on a hot spring trip together.