Synopsis
The Doctor and Rose materialise the TARDIS in a massive underground bunker near Salt Lake City, Utah in the year 2012, drawn to it by a distress signal. They find they are in the Vault, owned by the wealthy and greedy Henry van Statten who collects and houses alien artifacts within it. While Rose tours the facility with one of van Statten's technicians, Adam Mitchell, van Statten offers to take the Doctor to see the pride of his collection, the "Metaltron", contained in a special part of the Vault called the Cage. The Doctor is first shocked and then horrified to find the Metaltron is actually a Dalek, all of whom he thought destroyed in the Time War, but then laughs and ridicules the Dalek when he discovers it is weakened and chained down, unable to fight back. The Doctor attempts to destroy it, but is stopped by van Statten's guards and escorted back to his offices.
Meanwhile, Adam has led Rose to the Cage, after finding that the Dalek has been tortured. She takes pity on the Dalek. She touches its casing, which gives the Dalek the chance to absorb her DNA and time energy from the radiation she has been exposed to as part of her travels in the TARDIS. The Dalek uses these to become re-energised, drawing power from all over the southwestern United States to recharge itself, and breaks its bonds. The Dalek breaks out of the cage by decoding a billion combiantion lock and enters a computer lab. After learning from the Internet that it is the last Dalek, the Dalek begins shooting all the guards in its way. It pursues Rose and Adam to a stairwell where it elevates up the stairs after them. A big room full of guards and troops fire at the Dalek but The Dalek destroys a fire alam and electrocutes all of the soldiers. The Dalek then communicates directly with the Doctor, explaining its ability to regenerate by means of DNA extrapolation when Rose touched it. However, as the last remaining Dalek and without a command structure to receive orders from, it is, to all intents and purposes, useless. The Doctor then suggests the Dalek kill itself to remove the presence of its race from the universe. However, the Dalek refuses, stating "The Daleks must survive" and stating that it will follow the primary directive of all Daleks to exterminate non-Dalek life from the universe. The Doctor becomes increasingly aggressive, demanding it destroy itself, but the Dalek ignores him, coldly stating "You would make a good Dalek" before cutting communications.
Adam tries to help Rose escape, but she is trapped inside the Vault when it is sealed by van Statten. The video feeds are disrupted moments before the Dalek encounters Rose, infuriating the Doctor. The Dalek, infused with human DNA, shows sympathy for Rose and spares her life for the moment, demanding its release from the Vault in exchange for Rose's life, which the Doctor feels forced to allow. The Dalek makes its way to van Statten's office seeking to kill its erstwhile captor, but Rose intervenes, eliciting from the Dalek an indication of what it really seeks: its freedom.
The Dalek and Rose go to the highest point in the Vault, and the Dalek shoots a hole in the ceiling, allowing sunlight into the facility. As the Dalek opens its armoured casing and experiences the sunlight directly, the Doctor appears with an alien weapon, ready to destroy it. Rose again intervenes and deters the Doctor, showing him that her DNA has caused the Dalek to mutate and is no longer able to perform its objective. After asking Rose to order it to do so, the Dalek annihilates itself, as it believes anything different to a Dalek is wrong and that it was not experiencing life, but sickness. In the episode's dénouement, van Statten is deposed by his aide, who orders him mind-wiped, and then dumped on the street. Furthermore, the Vault is to be filled with cement and sealed off. At the TARDIS, the Doctor ruefully observes that as the last survivor of the Time War, he "wins". He also tells Rose that he would be able to sense the presence of other Time Lords had they survived. As Adam warns them about the impending closure of the Vault, Rose invites him to join them in the TARDIS, much to his bewilderment.
Read more about this topic: Dalek (Doctor Who Episode), Plot