Doomsday

The episode's opening continues from the final scene of "Army of Ghosts"; Dr Singh, Mickey Smith and Rose Tyler are trapped in a sealed room within Torchwood Tower. Four Daleks, accompanied by a device known as the "Genesis Ark", have emerged from the void ship. A Dalek Supreme called Dalek Sec extracts information about Earth from Singh, killing him in the process. He discovers that a separate invasion is in progress, and sends Dalek Thay out to investigate. The Cybermen, who took control of Torchwood, detect the Dalek technology and offer an alliance. The Daleks decline, and the two species declare war on each other.
While discussing humanity with the Doctor, the Cyber Leader is destroyed by a strike team led by Jake Simmonds, from the same universe as the Cybermen. Jake takes the Doctor to his universe and a parallel Torchwood. Pete Tyler tells him that the Cybermen vanished and they have followed them. But the breach is causing the parallel Earth to boil and must be closed, before the damage destroys both worlds. They return to Earth and approach the Cybermen with a truce against the Daleks.
Meanwhile, Rose tells Mickey they were kept alive because, as time travellers, their touch would activate the Ark. Sec explains that the Cult cannot open the Ark because it is stolen Time Lord technology. He demands that Rose open it, but she refuses, and goads the Daleks with her knowledge of the Time War's conclusion and the fact she annihilated their Emperor until the Doctor appears. Upon realising that the Daleks are the Cult of Skaro, he uses his sonic screwdriver to allow the Cybermen to attack the Daleks, letting the humans escape. Mickey accidentally activates the Ark when escaping, and the Daleks, after fighting off the Cybermen, travel to the exterior of Canary Wharf to release the Ark's contents: millions of Daleks who were imprisoned during the Time War. The Daleks descend on London, killing all in their path.
The Doctor and his companions flee into the tower. En route, Pete rescues Jackie from upgrading and the couple are reunited. As the Dalek-Cyberman war rages outside with humans getting massacred in the crossfire, the Doctor brings everyone to the breach room. He explains that crossing the Void causes a traveller to become saturated in Void material. If he opens the breach to the void, any being saturated will be pulled in. He will then close the breach forever. However, Rose and Mickey have crossed the Void and are contaminated and vulnerable.
The Tylers and Mickey cross into the parallel universe, but Rose decides she would rather be with the Doctor than her family and jumps back to help him. They open the breach and hold onto a pair of magnetic clamps as the Cybermen and Daleks are pulled in, though Dalek Sec and the Cult of Skaro escape via an "emergency temporal shift". Rose's lever slips. She pushes it back into position but loses her grip. Before she falls into the Void, Pete reappears, catches her and jumps back to his world. Moments later, the breach slams shut forever, leaving a devastated Rose trapped in the other world.
Some time later, Rose has a dream where she hears the Doctor's voice calling her. The Tyler family follow the voice to a remote bay in Norway called Bad Wolf Bay, where an image of the Doctor appears, which he is harnessing the power of a supernova to transmit through one of the final breaches. Because the breach is to close permanently in two minutes, the pair share one last conversation. Rose breaks down in tears and tells him that she loves him, but as the Doctor starts to reply, the breach closes. In the TARDIS, a tearful Doctor regains his composure and sets a new course. He looks up to see a woman in a wedding dress, who demands to know where she is.

Army of Ghosts

The episode begins with Rose Tyler narrating how her life changed when she met the Doctor, and that she thought it would last forever. But then came the ghosts, Torchwood, and the war, and begins to tell how her travels ended.
The TARDIS materialises on the Powell Estate so Rose can visit her mother, Jackie Tyler. Jackie shows the Doctor and Rose, to their surprise, that ghosts have been regularly visiting since they last left. After watching various television programmes, the Doctor and Rose find out that the ghosts started visiting several months before, and over time humanity accepted them. Disturbed by both the ghosts and their appearances being scheduled, the Doctor traces the "ghost shift" to One Canada Square, the secret base of the Torchwood Institute.
Torchwood are shown to be, along with causing the ghosts to appear, experimenting on a sphere, which arrived concurrently with the ghosts. The sphere is commented to be without mass or a gravitation field – physically, it does not exist. The cause of all these is shown to be a breach between universes caused by the sphere, allowing Cybermen from a parallel universe to filter through, and slowly infiltrate Torchwood's ranks.
Upon arriving at Torchwood, the Doctor is taken prisoner, along with Jackie, who Yvonne Hartman, director of Torchwood, believes to be Rose. The Doctor is told that his encounter with Queen Victoria made him an enemy of the state, and was the catalyst for the creation of Torchwood. He is then shown the sphere, which he identifies as a "void ship", and the experiments on the breach, which the Doctor strongly disapproving of both practices, noting that experimenting with the breach will only cause it to fracture more.
Rose, free from detection, masquerades as an employee, and explores Torchwood. She is able to get into the chamber where the void ship is located, but is immediately questioned by Dr Singh, who is in charge of experimenting upon the sphere. He discovers that Rose is an imposter, and asks his colleague Samuel, who Rose recognises as Mickey Smith, to seal the doors.At the end of the episode, the breach is fully opened, and the multitude of Cybermen are able to cross the Void and take over the Earth. At the same time, the void ship begins to open, and Mickey produces a gun to fight the Cybermen he expects to find inside. But the Cybermen tell the Doctor that they merely followed the sphere across worlds and do not know its origin. When it opens fully, four Daleks emerge, identify the Earth and vow to exterminate its lifeforms.