Ep 4. The Sontaran Stratagem

In season four, episode five of Doctor Who (2005), The Sontaran Stratagem, the Doctor and his companion, Donna, find themselves in Cardiff investigating strange activity at the ATMOS factory. The ATMOS system is being installed in cars around the world to reduce emissions, but the Doctor soon realizes that there is something more sinister going on.

The Doctor discovers that the factory is being run by the Sontarans, a militaristic race of aliens who are at war with the Rutans. The Sontarans plan to use the ATMOS system to release a poisonous gas into the atmosphere that will wipe out all alien life on Earth, including the humans.

The Doctor and Donna team up with UNIT, a military organization that deals with alien threats, to stop the Sontarans. They are joined by Martha Jones, a former companion of the Doctor's, who now works for UNIT. Together they come up with a plan to infiltrate the factory and stop the Sontarans.

However, the Sontarans are one step ahead of them and capture the Doctor. They use him to start the release of the gas, but the Doctor manages to escape and destroy the factory, saving Earth from the deadly gas.

In the aftermath of the battle, the Doctor is reunited with Martha and says goodbye to her before he and Donna set off on their next adventure.

The Sontaran Stratagem is a thrilling episode filled with action and adventure. It brings back an old companion and introduces a classic Doctor Who villain in the Sontarans. The episode also has a message about the dangers of technology and the importance of working together to stop threats to the planet. Overall, it is a must-watch for fans of the show.