The format works well and added depth to the story.
Aliens are coming, speaking through children and they've come to take our children or destroy us all. An exploration of family and of ethics via the medium of entertaining pacey drama. Plenty of spoilers here.
Our place in the universe. Fleetingly we hear that a Christian gives up her faith because science made her feel too insignificant in the universe. Torchwood exists in a godless universe where there are aliens who challenge our place. Does a big universe make us big or small? Does the scale of things tell us about us or about something else?
This is about children. Along the way we find that Captain Jack has a daughter, and a grandson, and Gwen is pregnant. Children everywhere. The question of whether she would have an abortion is very briefly tackled as we find her saying she wouldn't do that - while Jack has to sacrifice his grandson (and his relationship with his daughter) to save the world. He loses himself along the way and flees to find himself - was the greater good worth it or was this evil... Corrupt politicians are happy to sacrifice the children of others, while a civil servant takes the lives of his whole family to spare his children from being taken by the aliens. Aliens abuse children as narcotics. Gwen ponders the depravity of humanity, giving up its children, concluding that the Doctor must find it so terrible that he wont step in and save us. How are we to make our ethical decisions? What justice and cleansing is there for evil? Is there hope for us and for our children?