The Sontaran Stratagem is the fourth episode of the fourth series of the revival Doctor Who. It starred David Tennant as The Tenth Doctor, Catherine Tate as Donna Noble and Freema Agyeman as Martha Jones. It was the first part of a two part story, the second part being "The Poison Sky". As usual, I will treat the two parts as one story.
The Doctor is called back to earth by Martha Jones, former companion and current advisor for UNIT. It seems that there is trouble brewing: a system of automobile emissions control turns out to be something much more sinister, although just how sinister is only discovered when the Doctor realizes that the Sontarans are behind the plot, which also involves a mysterious and sinister private school/think tank for ultra-gifted youth. And then Martha Jones gets cloned and the sky turns to poison, and the Doctor has to defeat the Sontarans and save the world. That is a brief sketch of the plot, and for those familiar with Doctor Who, it all makes sense.
As I have written in earlier reviews of Series Four, this Series has been much more serious. And despite the fact that this is a dramatic two-part episode where the world is in danger of destruction, it is in some ways lighter fare than other episodes we've seen in Series 4. It is an action-packed epic with car chases and action scenes and The Doctor disabling a Sontaran with a pingpong ball. It is also has great character interaction, since it has The Doctor, Donna Noble and Martha Jones.
Although the episode does have its serious parts. One of the most telling is The Doctor, after having repeatedly spoken out against turning the crisis into a military conflict, shows some excitement when a great battle begins. The commander of the UNIT forces asks him whether he is beginning to like it, and The Doctor quickly replies that he isn't--- although it is quite clear that indeed, he is. This would be an important theme developed through Series 4 (and indeed, through all of both the Classic and Revival Doctor Who): The Doctor as healer versus The Doctor as Warrior.
Overall, however, this episode probably works best as an action-themed episode with good character interaction, although it is still more serious in tone and better written than many other of the other big two-part stories in the Revival Doctor Who.