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3. Mirror Chekov
Mirror Chekov is our first real shock that this is not just a rougher Starfleet, but a full-blown terror state where even junior officers are conditioned to murder their way up the ladder. Chekov's failed attempt on Kirk’s life is treated like a routine career move, and when it goes wrong, he’s dragged to the Agony Booth, a punishment device everyone accepts as standard operating procedure.
Seeing an otherwise cheerful navigator reduced to panicked screams sells the Mirror Universe faster than any exposition could. That moment also lets viewers see how normalized cruelty has become: guards watch his torture without flinching, and Mirror Sulu’s only concern is how the failure affects his own schemes.