This episode was always destined to be the most emotional of the new series, and after the more traditional 'Who' of The Long Game, it was to be a very different type of story. Rose never knew her father, Peter. He died when she was a baby. Brought up by her mother, she only knew her father from what she had been told. As far as she knew, Peter was a businessman, a loving husband, and a doting father. He died alone - the victim of a hit and run.

This background information has been lurking behind the scenes since the first episode, and was re-told - perhaps with more detail - skillfully by Rose, and in flashback by Jackie to her young daughter. Explaining to the Doctor why she's been thinking so much of her father quickly, and easily, explains to the viewer. Rose merely wanted to see her father when he was alive, to know what he was like, and - more importantly - she didn't want him to die alone. But, as the Doctor says, be careful what you wish for.

On seeing her father, and witnessing his death, Rose is incapable of action. She asks the Doctor for a second try. At this point the story could so easily have become Groundhog Day, but fortunately Phil Cornell must have been aware of this and stipulated that they could only be there twice. On this second attempt to be with her father at his death, Rose realises that she can't face losing him again, and acting purely from grief she rushes out to save his life. Who amongst us can say, hand on heart, that we wouldn't do the same? The catastrophic results which ensue from this single act lead to the end of the world, and inevitably Peter realises that he is the only person who can save the world - the only person who can make things right. Along the way we witness tempers, and tantrums, from Peter, Jackie, Rose and the Doctor, and Rose discovers the truth about her parent's relationship. Disappointed, and disillusioned, she watches them bicker. Rose is overflowing with remorse that her selfish actions have led to such a catastrophic turn of events, and again she has to face the death of her father. But this time she has a chance to say goodbye, as does Jackie, and Peter chooses to die - rather than being a victim, he becomes a hero, someone Rose can be truly proud of. Peter recognises that the Doctor knew the truth, that he knew that Peter had to die - but was trying to find a better solution. In the end, Peter doesn't die alone. Rose is with him, and she comforts him in his final moments. She attains closure, and, in two short scenes, we see the truth of the strong bond she has with the Doctor - his actions and her father's mirrored when comforting her.

This episode was wonderfully written, fully exploring the 'what if?' scenario, which we all face when dealing with the grief of losing a loved one. Can anyone truly say that this element of time-travel has been so well explored since H G Wells? Once again, we see a more human side to Eccleston's Doctor, and Chris was - as ever - superb. The Doctor's anger at Rose's stupidity - from the simmering silence to "I picked another stupid ape" - and his attempt to protect her, and her family, and his self-sacrifice, were portrayed in a wonderfully understated way. All of these emotions truly convey how much the Doctor cares about the human race, perhaps the most moving speech was when the Doctor was talking to the couple who were about to get married, ending with "I never had a life like that" shows us just how much the Doctor lost in his years of exile.

Camille Coduri, and Shaun Dingwall, were excellent as Rose's bickering parents, who recognised that they still loved each other intensely at the end of the episode, and who's courage and self-sacrifice poignantly saved the world. I felt their grief, and pain, and the mental anguish that Pete was going through was so well portrayed. But the praise really has to go to Billie Piper. This is certainly Rose's hardest adventure to date. She doesn't realise how hard this will be, and she faces her emotions head on. Two weeks ago Chris portrayed the Doctor in a highly emotional state, believing that he'd killed Rose. This week Billie portrayed Rose in a similarly emotional state, believing that she's killed the Doctor. On top of the grief, and remorse, which Rose was already dealing with, Billie made us feel that this really was the worst day of Rose's life. When Rose hugged her father, as he realised who she was, I felt her pain and grief - and this is all testament to Billie's superb acting. She has been a revelation in this series, and this episode has surpassed all others in terms of her acting - I'll never doubt her again.

At the end of this episode, I was left wondering what I would do if I had a TARDIS - how, or if, I would fight the temptation to do something similar and save my mum's life. And I know, deep down, that like Rose I'd meddle with history. In that situation who wouldn't? And I cried. I cried when I watched a second time, that's how strong the story is. That's how good the acting is. This is twice now that 'New Who' has made me cry. And this finally proves that great Sci-Fi can also be great Drama.