Conspiracy

A  was a plan, usually some politically scandalous act, made in secret between a select group of individuals, who were entrusted to tell no-one else.

In Ancient Rome, the Doctor appeared to sense some sort of conspiracy, but could not identify it until Vicki causally let slip that she had been involved in a plot with Maximus, Tavius, and Locusta to poison Nero. The latter two of these were pretending to work for Nero while plotting against him. Nobody else outside of this group was supposed to know of these plans, officially, and Vicki seemed to panic one she realised what she’d done. Despite agreeing with the plotters on Nero’s evil, the Doctor was forced to scupper their plans and save Nero’s life in order to preserve the space-time continuum, since Nero’s death could not be rewritten. Nero tricked a slave into drinking the wine to see if the Doctor was telling the truth, and received confirmation when the slaves, choking, collapsed down dead. Unaware of these motivations, the other plotters believed the Doctor to be a friend of Nero following this incident, and deemed Vicki untrustworthy for telling him. (TV: "Conspiracy")

It was something on which the Thirteenth Doctor's view alternated. Though she initially claimed to "love a conspiracy", (TV: Arachnids in the UK) she later said that she did not like conspiracies. She went on to say that the problem with conspiracies was that there was so much to think about. (TV: Kerblam!)

Nazi leader Adolf Hitler responded in his suicide note to allegations of starting World War II by claiming have been provoked into it by a Jewish conspiracy (PROSE: The Shadow in the Glass). However, the defence did not appear to be taken seriously, and most accounts placed the blame for the war squarely at his feet (TV: Silver Nemeis, Let's Kill Hitler, PROSE: Timewyrm: Exodus, The Spear of Destiny)

Belief in a conspiracy, whether a real or imagined one, was known as a ‘conspiracy theory’. Many of these were published on the internet, inspiring Donna Noble to investigate Adipose Industries (TV: Partners in Crime), and Clive Finch to keep records on the Doctor, mostly focusing on his ninth incarnation, (TV: Rose) but with a few photographs of other incarnations of a nature that was unknown. (PROSE; Rose)