Over the course of the Break on Through story mission of Call of Duty: Black Ops Cold War, you’ll grow annoyingly familiar with a red bunker door. Adler really wants you, as the character of Bell, to go through that door to learn more information about the villainous Perseus. This mission will have you go through a fabricated Vietnam memory repeatedly, and there will be plenty of opportunities to obey or disobey Adler’s wishes — the only opportunity that matters to obtain the achievement or trophy called The Red Door is the last one.
You will go through this sequence of events three different times, albeit with variations each time. For the fourth run and final, Adler will become very impatient, and he will skip the first part of this flashback in which you fight the Viet Cong. Instead, you’ll be placed in front of a familiar temple ruin. Because Adler just wants you to go through the damn door, he’ll place it right inside the temple for you to see.
We’re not here to obey orders, though. Instead of going straight to the door, go either left or right of the temple ruin and skip the door entirely. Adler will obviously not be happy about this. In this weird, twisted memory mindscape of yours, something rather cool will happen. As Adler tries to get you through the door, he’ll spawn more red bunker doors from the sky, and they’ll drop right in front of you. Keep passing by these doors and don’t go through a single one.
Eventually, Adler will be fed up. In a flash, you’ll be teleported to an enclosed space where you’ll have no choice but to enter the red bunker. Interact with it to proceed to the mission’s ending cutscene, but even by then, the achievement should pop up.