When we last left the Tenth Doctor and Rose, the Cybus species of Cybermen from an alternate universe-using the technology of Torchwood-had broken into our universe and begun in an invasion. However, a group of four Daleks-the Cult of Skaro-had likewise showed up, having survived the time war in a “void ship”. They also carry with them a mysterious device known as the “Genesis ark”.
The two cyborg-races almost immediately notice each other. The cybermen attempt to negoatiate a truce, but the Daleks-being what they are-won’t tolerate any alliance. Naturally, war breaks out-despite the Cybermen vastly appearing to outnumber the Daleks, the Dalek’s defensive technology is superior-and we soon find out that there’s even more Daleks, as the genesis ark is, in fact, a time lord prison carrying hundreds of Daleks, kept in a small space similar to the TARDIS.
As the war goes on around Torchwood, the Doctor and Rose team up with the alternate Pete Tyler, Mickey and their friends who have followed the Cybermen from their universe.
The Doctor figures they can use the portal to teleport the Cybermen and the Daleks into the void, a sort of ‘nowhere’ dimension. The technology is however unstable, and although the Doctor is able to suck the Dalek and Cybermen armies into the void (apart from the cult of skaro, who escapes using time-travel), Rose and Jackie are permanently sent to the alternate universe.
The Doctor’s won. However, he’s also lost.
The Doctor is able to project himself into Rose’s universe months later, using a dying star to power the projection. There’s no way she can come back to her own universe, although she and her family are happy (and Jackie is pregnant with alternate Pete’s son) The two share their last goodbyes, with Rose declaring her love for the Doctor (He’s about to say it too, but gets cut off at the last minute.
Back in the TARDIS, the Doctor cries, but he’s only got minutes to grief, as a strange looking woman materializes in the TARDIS.
Her name is Donna. She’ll become very important. But for now, the she and the Doctor have absolutely no idea what she’s doing there.