Oh what a great show this could have been, except it was an episode short! The new format of one storyline to an episode is IMHO destructive to character development and ultimately to Dr Who. When capable, intelligent actors are turned into shadow puppets with no substance then interest can but wane.
Quite a few holes in the plot and continuity. Like when the youngest brother gets a pole right through his shoulder and he then picks a fight with his brother when freed only to remember he is hurt after the fact (no blood and no hole). Also if the engine was based on a nuclear rod principle then he'd have been melted anyway.
Dr Who has got a lot to offer but not as a fast food meal with no substance and not as a science fiction show without the basic science included. If these flaws were a one offs in a good season I'd forgive and forget but as the whole season has been flawed I think the buck stops with Stephen Moffat. Come back Russell T. we never had to forgive you anything.
On the plus side some good acting by Matt and Jenna-Louise. Poor Jenna hasn't been treated very well up to now with her character reverting from cool, assertive adult to a silhouette of a 14 year old looking for the Dr's approval. Not her fault just poor scripts. Lets hope this episode is indicative of the old Clara's return.