Stranded on Earth for 20 years, the Master was forced to become the scientific adviser to the United Nations until he later defected to China when he discovered mind parasites which preyed on darker thoughts like anger that were the survivors of an alien spacecraft and had been hidden all over China.
Knowing that the Unbound Doctor frequently visited Earth, the Master planned to meet him so he could escape but he didn't realise that the Doctor had been caught by the Time Lords and placed on trial. When the Doctor arrived on Earth in the 1990s in Hong Kong before the British handed the city to the Chinese, the Master was trying to track down the last parasite in an aeroplane that had been adapted to become invisible using "a crude adaptation of a TARDIS chameleon circuit," but the plane crashed and the Master was so badly injured he was forced to regenerate.
In his new incarnation, the Master met the Doctor at last and angrily confronted him where he accidentally revealed the loss of his TARDIS. At the same time, the Master learnt the Doctor's own TARDIS had been locked by the Time Lords with an inhibitor. The Master was scornful and said "a child can fix that" before learning the knowledge had been locked out of the Doctor's mind.
The Master promised to help the Doctor in return for the destruction of the last parasite, but he planned to trick the Doctor into handing him the key to the TARDIS so he could make his escape alone. Unfortunately, the Doctor had counted on the betrayal and instead handed the Master the keys to the retired Brigadier Lethbridge Stewart's pub, the Little England, commenting on the camouflage as "good" because he believed it was a genuine TARDIS. When the Master reached the doors, he whispered "a new TARDIS, at last" before he went inside and discovered the truth and rushed back to the Doctor only to catch him and the Brigadier leaving in the TARDIS. (AUDIO: Sympathy for the Devil)