1)Another "base under siege" (or "ship under siege", if you prefer) this time around. Fairly basic stuff, but not bad. Supposedly the title was a gag on 24, which was an American television program that was being broadcast at the time.
2)The actual story itself is fairly standard, for Doctor Who, but we see the new show exploring areas that theold show never went into, that I can recall, i.e., Martha's sudden realization that if she gets killed out at the far end of spacetime, her familty would neve rknow what happened to her.
3)We also get more of the adventures of Francine Jones and the agents of Mr. Saxon. I noticed that at the end of the episode we get a brief flourish of what we do not realize is Murray Gold's "The Master" Theme. It has been pointed out that "Mister Saxon" is a anagram of "Master No. Six". I'll confess to not having picked up on that at the time.
Overall An enjoyable enough, tense little drama. Fun to re-watch.
[Part of list of Doctor Who episodes here.]