Complex sentence: The Doctor and Rose investigate and realise that the source of the problems is a solitary 12-year old girl, Chloe Webber, who is able to cause people to disappear by drawing them.
Split into simpler sentences: The Doctor and Rose investigate and realise that the source of the problems is a solitary 12-year old girl named Chloe Webber. She has the unique ability to cause people to disappear by drawing them.