The Sontaran Strategem / The Poison Sky
This is maybe not quite an all time classic amazing story but it is a strong and entertaining two parter of very good standard.
The first part features the Doctor and Donna meeting up with former companion Martha and UNIT, who she now works for, to look into ATMOS, a company whose widespread satellite navigation systems have been causing deaths. They are correct to suspect alien interference as the Doctor's old adversaries the Sontarans are using ATMOS as a way to effect the atmosphere and invade the Earth.
There is excellent comedy, plenty of action and smart dialogue. Tennant, Tate and Agyeman as the returning Martha are all as great as ever and the wonderful Bernard Cribbins gets involved as the fantastic Wilf. The character development for Martha is superb as she moves on from her crush and her inexperience when she was first with the Doctor. She is now a strong, independent leader as well as an intelligent, brave and capable young woman.
UNIT are less impressively portrayed I feel with officers who are just a little bit cliche daft army people but there are a few convincing and well written young soldier characters.
The young genius and his team behind ATMOS are not fully convincing to me at times but they are perfectly adequate, especially in the 2nd episode.
The Sontarans manage to bring amusement and enjoyable alien threat and their new incarnation is very good I think.
Martha's clone works well and gives an extra side for Freema Agyeman to show us. Meanwhile Donna is funny and has great humanity.
The cliffhanger between episodes is great.
Not perfect perhaps but a very fun adventure.
My Ratings:
The Sontaran Strategem - 8.5/10
The Poison Sky - 9/10
Overall rating for both parts - 8.75/10.