A World War I soldier recuperating in St Teilo's is the key to saving the world. 1918 and 2008 are intersecting, flooding into each others time and threatening to destroy the world - yadda yadda yadda. Heard it all before?
Well in all honesty you have - there is even a Torchwood season one episode that is very similar in plot. But it is well done and that rescues it. There is some genuine tension in some of the "ghost" scenes, and the human, emotional element is handled reasonably well without feeling too cloying.
Three episodes of the season down and the best so far, but then I like time paradox stories.