Memory bomb

The Dalek Combat Training Manual, published during the Last Great Time War, claimed that a memory bomb had been concealed within the control console of the Doctor's TARDIS. The First Doctor, potentially under coercion by the Celestial Intervention Agency, had taken the Hand of Omega with him when he escaped from Gallifrey. The Doctor placed the Hand on Earth in the 20th century, aware that the Daleks would come to steal it in the hope of harnessing its great power. Accordingly, he pre-programmed the Hand to act against Dalek interests. Ultimately, the Doctor's scheme was delayed when he unexpectedly left the planet Earth, and the memory bomb wiped his mind of knowledge of both the Daleks and the Hand of Omega as insurance against his plan being discovered. (PROSE: Dalek Combat Training Manual) As a result, the Doctor was unfamiliar with the Daleks until he encountered them on Skaro. (TV: The Daleks, Twice Upon a Time) The Doctor's memory of the plan itself would not resurface until his seventh incarnation, who would return to Earth to execute it. (TV: Remembrance of the Daleks, PROSE: Dalek Combat Training Manual)