While spying on the Doctor, the Master inadvertently enabled two way communication and unwittingly revealed his plans to eliminate the Doctor. After the two had arranged to meet on Tersurus, it became apparent the Master had travelled back in time to bribe a Tersuron architect to install traps in the castle they were now in. Unbeknownst to him, the Doctor had also done the same, bribing the architect with food to change the traps' details. This led to the Master floundering around the sewers of Tersurus for over nine centuries, after falling down the same trap three times. Later he allied himself with the Daleks, and then used his very own TARDIS to travel back to the point just after he fell. Then he threatened the Doctor once again with the Daleks, as they were the only life-forms without noses and hence the only creatures willing to work with him.
As part of this alliance, the Master had himself "augmented" with Dalek technology, resulting in a more youthful appearance and "Dalek bumps" on his chest. Although he remained adamant they were etheric beam locators, the Doctor continually teased the Master because they resembled breasts.
When he believed the Twelfth Doctor to have been killed by the zectronic beam, the Master renounced his evil ways before the Doctor regenerated into a female form. Attracted to each other, the Time Lords walked off, arm in arm. (TV: The Curse of Fatal Death)
This version of the Master was as melodramatic and hammy as ever, frequently laughing maniacally and taking pleasure in explaining his evil plans at great length.
He was also significantly more inept than other versions of the Master, monitoring the Doctor in his TARDIS and gloating about the traps he had set without realising that he was on speakerphone until the Doctor informed him. After deactivating his viewscreen, he continued to monologue in what he thought was privacy, unaware that he had just switched the screen off and was still audible to the Doctor. He also inadvertently fell into a trap he had set for the Doctor no less than three times.
The Master's incompetence manifested itself again when he failed to realise that the Daleks were planning to exterminate him once had assisted them in completing the death ray. In fact, so lacking in common sense was the Master that when the Doctor used the language of the Tersuruns to discreetly (and pungently) inform him of the Daleks' intentions, the Master gave the game away by blurting it out loud, causing the Daleks to try and exterminate him post haste. This Master's uncharacteristic lack of intelligence was further demonstrated by his inability to repair the exploding generator, admitting that only the Doctor had the capacity to do so. (TV: The Curse of Fatal Death)