The Doctor takes Rose back to when her father is killed on the way to a wedding. Rose acting on impulse runs and saves her father from being killed. This has an affect on the timeline changing the future. Reapers, giant winged serpents appear through a dimensions portal, a side-effect of changing what has happened. Everyone takes sanctuary inside a church whilst The Doctor works out what to do.
Rose was always bought up being told how great her father is but the reality is different from what she's told. Rose is a baby in those days. Her father and mother do not see eye to eye on anything. Faced with being time-locked, her father notices a car appearing and disappearing, he realises to get time back to normal, he must die. He leaves the church and walks in front of the car thus killing himself.