The Master in Doctor Who is defined by a deep-seated madness caused by constant drums in his head. The "Harold Saxon" Era: He infiltrated human society, was elected Prime Minister of the UK, and took over the Earth using alien Toclafane. He was ultimately defeated by the Tenth Doctor and Martha Jones, but maliciously refused to regenerate after being shot by his wife.The End of Time: Resurrected in a frail, unstable state by a cult, he attempted to rewrite the entire human race into copies of himself. He was sent back to the Time War but managed to survive. John Simm's Master is a manic, chaotic, and dangerously unhinged Time Lord. Often described as the dark, unrestricted mirror to the Tenth Doctor, he blends fast-talking charisma with a cruel, power-hungry ego. His instability is primarily driven by a never-ending drumbeat echoing inside his head. The Anti-Doctor: Simm’s portrayal is widely noted by critics as the ultimate "Anti-Tennant". He is fast-talking and highly energetic, but unlike the Doctor—who uses these traits alongside a heavy conscience—the Master has no moral boundaries and revels in absolute villainy.Joker-esque Chaos: Moving away from the stoic, organized villains of the classic series, this incarnation leans heavily into a highly volatile, Joker-like agent of chaos. He delights in dancing, laughing hysterically, and toying with his victims.Driven by a Drumbeat: The defining tragedy and source of his madness is the "four knocks"—a constant, rhythmic drumming implanted by the Time Lords in his childhood. This drives his desperate obsession with power, survival, and asserting control over others.Megalomania and Narcissism: Rather than operating purely from a classic enemy-rival dynamic with the Doctor, his schemes (like taking over Earth as Prime Minister "Harold Saxon" or forcing every human to become a clone of himself) stem from a deeply selfish desire for exposure, validation, and control.