S04E05: 'The Poison Sky'

Sylvia saves Wilf by using an axe to break into the car. The Doctor gives Donna a Tardis key and she goes inside, only for the Sontarans to beam the vessel onto their spaceship.

UNIT are keen to launch a missile strike against the Sontarans, but the Martha clone keeps discreetly preventing it. Via a video link, The Doctor addresses General Staal and slyly gives hints to Donna that she should phone him.

Luke Rattigan's pupils flee his academy after he reveals plans to transport them to another planet with the aid of the Sontarans. He had already devised a mating programme for them. The young genius then discovers that he has been betrayed by the Sontarans as they only wanted him and his pupils for target practice.

Sontaran soldiers attack UNIT troops at the Atmos factory and find the battle easy, as the Earthlings' guns don't work. The Doctor's new soldier friend Ross is killed by the aliens.

The Doctor guides Donna through the process of reopening the teleport link on board the Sontarans' ship, while back on Earth UNIT use the Valiant to help clear the smog. The Martha clone is exposed by The Doctor and subsequently dies after he rescues the real Martha.

At the Rattigan Academy, The Doctor uses Luke's atmosphere converter to clear the world's skies. In a bid to stop the Sontarans from destroying Earth, the Time Lord teleports on board their ship with a bomb and threatens to blow up everyone on board - including himself - unless the aliens leave in peace. However, Luke uses the teleport device to beam on board the Sontaran ship in the place of The Doctor and detonates the device.

The Doctor prepares to say goodbye to Martha in the Tardis, but the doors suddenly close and the vessel is transported to an unknown destination.

> Click here for our review of 'The Poison Sky'