Given the popularity of the first Dalek story, it was inevitable that the popular adversaries would be making a return appearance and it certainly doesn't disappoint.
The Tardis lands in late 22nd century London, which they find in ruins. The Doctor and Ian go off to explore while Susan and Barbara remain behind with the ship. Susan and Barbara are captured by human resistance fighters, while The Doctor and Ian are captured by the Daleks and their human slaves The Robomen.
The Doctor is due to be converted into a Roboman, but an attack by the resistance fighters foils the Daleks plans. After this attack, the Daleks retreat from London to their mining operations to the north of London in Bedfordshire.
The Doctor, Ian, Barbara and Susan each make their way to Bedfordshire. Ian stows away on the Dalek ship, along with Craddock, one of the resistance fighters. Barbara and another resistance fighter, Jenny, make their way first by truck and then on foot. The Doctor and Susan arrive with the help of David Campbell, Carl Tyler and other resistance fighters.
Once in Bedfordshire, the Daleks' plans are made clear. The Daleks intend to remove the core of the Earth, which would allow the Daleks to use the Earth as a massive spaceship. The Doctor, with the help of his companions and the resistance fighters eventually foils their plans by turning the Robomen on the Daleks.
One does have to wonder why the Daleks would need the Earth as a spaceship. That particular plot point is never really explained.
Unlike most of the stories from the first series/season, The Doctor fully embraces the need to fight and resist the Daleks. The Doctor has gone from one fighting simply for self-preservation and for a higher goal, the defeat of evil. This is spelled out explicitly at one point early on in the story, when after being captured by the Daleks, The Doctor tells Ian that they must fight and defeat them.
The Dalek Invasion of Earth also witnesses our first companion departure. Susan, The Doctor's granddaughter, falls in love with David Campbell, one of the resistance fighters. Susan, at one point, tries to convince David to join the Tardis crew and travel with her through time and space. David, however, is adamant about staying and trying to rebuild the Earth and implores Susan to stay with him. The Doctor, realizing the tough decision that Susan is facing, makes the decision for her by locking Susan out of the Tardis.
I think the way the show handled the departure of Susan, with a very emotional farewell, certainly did a great deal to allow viewers to accept Susan's departure and to embrace the new companion, Vicki, who would be introduced in the next story. It also got viewers of Doctor Who used to the idea that change would be only constant in the Doctor Who universe and in many ways helped to pave the way for the changes that would eventually come in the future.
