Umi Sasamoto was in a traffic accident in her second year of high school and fell into a coma.

When she wakes up 16 years later, she has a strange sister who is the same age as she was before she went to sleep.

Umi is welcomed into her strange home with no recollection of her past.

Feeling a strong sense of incongruity at having the truth of the incident covered up, she tries to trace her fragmentary memories

(from mangaupdates)