Exxilon Gambit

The Exxilon Gambit was the final action of the Third Dalek War. (PROSE: The Secret Lives of Monsters) It took the form of numerous expeditions by opposing parties to the planet Exxilon to acquire the rare mineral of parrinium. (TV: Death to the Daleks)

Origin
During the Third Dalek War, (PROSE: The Secret Lives of Monsters) the Daleks launched a form of germ warfare, infecting the Outer Worlds of the Earth Empire with space plague. The colonists living on the Outer Worlds discovered that the plague could be cured by parrinium. Though extremely rare on Earth, it existed in abundance on Exxilon. The Marine Space Corps dispatched a small team on an expedition to Exxilon to acquire the parrinium and save millions of lives.

Though the Earth Empire was at war with the Daleks, they did not seem aware that the Daleks themselves were the instigators of the plague. The Daleks, knowing of the expedition to Exxilon, sent their own squad to the planet, tasked with acquiring the parrinium before the humans did (TV: Death to the Daleks) in an attempt to turn the tide of the war. (PROSE: The Secret Lives of Monsters) Another account noted that it was possible that Dalek-controlled worlds had also become infected with the plague. (PROSE: The Whoniverse)

Alliances of convenience
Upon arriving on Exxilon, the Earth expedition became stranded as a result of the Great City of the Exxilons drawing away power from their ship. The crew was attacked by the native Exxilons. Some of their number were killed and Commander Stewart was badly injured. His second-in-command, Captain Richard Railton, took over as leader while waiting for a relief ship. When the Daleks arrived, their saucer also fell victim to the power failure. With neither side possessing advanced weaponry (even the Daleks' gunsticks no longer worked), the Daleks and the humans, at the insistence of the Third Doctor, who was also stranded, were convinced to work together for the duration of the power failure. The Daleks kept their true intentions a secret and claimed they were also victims of the plague.

Railton was killed by the Exxilons and the combined expedition surrendered to Exxilon captivity. Though Stewart wanted Lieutenant Peter Hamilton to take command, the Commander died before he could enforce it and Lieutenant Dan Galloway took charge instead. Dalek reinforcements arrived with functioning projectile weaponry and blackmailed the Exxilons into providing slaves for a parrinium mining operation, to be overseen by Galloway. In return, the Exxilon High Priest asked only that the Daleks and Marines help them eliminate the splinter group of Subterranean Exxilons. The Doctor was distanced from the standing alliance after attacking the High Priest to save Sarah Jane Smith. The two made contact with the Subterraneans.

Attack on the Great City
The Doctor agreed to help the Subterraneans free the Exxilons from the influence of the Great City, which was blamed for the fall of the advanced Exxilon civilisation. The Daleks also began planning to assault the city once it was identified as the source of the power failure and after its defences attacked Daleks and Exxilons at the dig site. The Doctor and Bellal ventured into the City, as did two Daleks on a separate mission, to interfere with its functions from within. Meanwhile, the Dalek leader recruited Galloway and Hamilton to attach explosive charges to the City's beacon from the outside. The twin actions led to the destruction of the City, and restored power to the expeditionary vessels. This also meant the end of the truce.

Recovery of the parrinium
While most of the attention was focused on the City, Sarah Jane Smith and civilian scientist and geologist Jill Tarrant were able to store the mined parrinium aboard the Earth ship while replacing the Daleks' supplies with bags of sand. When the Daleks took off, they prepared to bombard Exxilon with plague missiles. However, Galloway took off with them as a stowaway and detonated a remaining explosive charge, obliterating the Dalek saucer. With the parrinium secure, Hamilton and Tarrant took it back to the Outer Worlds. (TV: Death to the Daleks)

Aftermath
The expedition constituted the last acts of the Third Dalek War. (PROSE: The Secret Lives of Monsters) Although the Daleks were defeated, the cost for Earth had been high, especially when combined with that of the Human-Draconian War and the Second Dalek War before it. The Earth Empire's vast population growth was brought to and end and with it, so was its expansionist phase. (PROSE: The Chase)

On the destruction of the Exxilon City, the Doctor considered it "rather a pity, in a way," as it meant the loss of one of the Wonders of the Universe. (TV: Death to the Daleks)

The events on Exxilon were recorded in a book by Hagan Garsonmous entitled A History of the Dalek Wars. (PROSE: The Secret Lives of Monsters) Another book about universal history seemed to attribute the plague and the Exxilon expedition as stemming from to the Dalek conflict that was sparked in the year 4000 by the attempt to develop the Time Destructor. However, its wider account seemed to offer a more general and not-necessarily linear account of the Dalek Wars as a whole. (PROSE: The Whoniverse)

From the Doctor's perspective, this was among his last encounters with the Daleks before the Time Lords sent his fourth incarnation to Skaro to interfere with their creation, after it was foreseen that there would come "a time when they will have destroyed all other lifeforms and become the dominant creature in the universe."

For the Doctor's companion, Sarah Jane Smith, Exxilon marked her first encounter with the Daleks, who were introduced to her by the Doctor as "living, bubbling lump[s] of hate." She was still in the Doctor's company at the time of his mission to Skaro. By that point, her experience with the Daleks, who she would describe as "the most evil creatures ever invented", left her with enough of an impression that she urged him to destroy them at their beginning. (TV: Genesis of the Daleks) Decades later, when faced with a full-scale Dalek invasion of Earth, she broke down in tears out of fear for the life of her son, Luke Smith. (TV: The Stolen Earth) Nevertheless, she would join with the Tenth Doctor and the Children of Time to defeat the Daleks once more. (TV: Journey's End)

Some of the Daleks survived, but were classified as insane and taken to the Dalek Asylum where they were stored in intensive care. The Eleventh Doctor encountered these Daleks when he was sent to the planet by the Parliament of the Daleks to help destroy it before the prisoners could escape. (TV: Asylum of the Daleks)

Behind the scenes

 * A History of the Universe and aHistory arbitrarily date the events of Death to the Daleks to 2600 as it takes place after (or at the end of) the Dalek Wars. Likewise, The Dalek Handbook dates these events as taking place during the 27th century. Given that the Second Dalek War of the 26th century began circa 2540, (TV: Frontier in Space) lasted around 40 years (PROSE: Prisoner of the Daleks) and took place in the same period as the Third, (PROSE: The Chase) this dating seems reasonably accurate.