Note about episode numbering: the ABC calls this episode 10. Others call this episode 11. The BBC website calls it "Season 7 Part 2 Episode 5".
(sigh)
Hmmmm. Very tense, this one. A "trapped with a monster" episode; this one different because it's the TARDIS they're trapped inside of.
( spoilers will be incinerated )When one steps back and thinks about it, there are a number of holes in the plot, but I was feeling generous and didn't let it bother me too much.
Mind you, if you compare it to another "trapped-in-the-TARDIS" episode, "The Doctor's Wife", it only gets a C+. Darn.