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.