Taken from Wikipedia:

Kirara is a young woman who, running late for her wedding, speeds in her car and dies in a car accident. Without understanding why, she finds herself transported eight years into the past, as a ghost. Kirara keeps her appearance, but can move through the air and can pass through walls at will. Kirara finds Kompei, her future fiance, and the Kirara of that time, who at that point are not a couple yet. The ghost Kirara cannot accept her separation from Kompei and tries to keep him to herself, at the expense of her past self.