Earth is attacked and Enterprise returns home at the request of Starfleet.
This episode feels like a prologue to a lot of television time dedicated to a big conflict between Enterprise and a new enemy. It introduces some concepts that do not really fit in to what has been established or referenced already within the Star Trek universe. That being said, it is pretty strong episode containing themes that seem relevant to the series which was written in the immediate post 9-11 years.
What it does well is generate intrigue into what will happen in the next. We see the Enterprise starting out on a seemingly dangerous journey and I for one want to see where they go and what obstacles they must overcome on the way. The sub-plot involving the Klingon Duras creates plenty of excitement before the Enterprise even enters the expanse.
Where it fell down a bit for me was the characters. There is a very big deal made about T'Pol either staying with Enterprise or returning to Vulcan. Let's be honest, she is one of the main characters on the show, so there's obviously only one way that's going. Trip Tucker, bereaved and angry following the death of his sister shows signs of how annoying he may possibly become in future episodes. Hoshi is virtually ignored considering she was only brought on as a Klingon translator.
I guess this is where the writers decided to go full hog and depart with both the continuity and formula of the past franchise. I understand from all the negative reviews of the next seasons that die-hard Trekkies hated it, but I'm happy to see for myself what all the fuss was about.