How Did Shepard Not Get Indoctrinated?

The games never explain it, because they never treat Shepard as immune. Commander Shepard runs into Reaper technology again and again, but only for short spells, and indoctrination in Mass Effect is slow: it wears a mind down over days of exposure. The one big exception, the idea that Shepard was being indoctrinated during the final mission of Mass Effect 3, is a fan theory that BioWare never confirmed. (Mass Effect; Mass Effect 3)

How indoctrination works#

The first game sets the rules. A Reaper, or a piece of its technology, gives off a field that works on the mind of anyone nearby. Victims get headaches, hear things and grow paranoid, and after long enough they simply obey. Saren explains that the more completely a Reaper controls someone, the less useful that person becomes. Matriarch Benezia held part of her mind back from Sovereign, but only for a while, and she tells Shepard she'll never be herself again. (Mass Effect)

That's the key point. Indoctrination isn't a single hit. It builds with time spent near the source, and the strongest minds only slow it down. (Mass Effect; Mass Effect 3)

Every time Shepard got close#

Shepard's exposure is real but short. On Virmire the Commander speaks with Sovereign directly through a device in Saren's base, then leaves. In the Arrival add-on for Mass Effect 2, Dr. Amanda Kenson shows Shepard Object Rho, a Reaper artifact her team has been studying. Shepard asks how they avoided indoctrination, and Kenson says they've been careful. They haven't: the artifact throws Shepard a vision of the Reaper fleet and knocks the Commander down, and Kenson and her staff turn out to be indoctrinated. Shepard escapes, destroys the relay and tells Admiral Hackett afterwards that the visions have stopped. (Mass Effect; Mass Effect 2: Arrival)

The contrast is the answer the games actually give. Kenson's team lived beside Object Rho. Shepard was in the room for minutes. (Mass Effect 2: Arrival)

In Mass Effect 3 the closest Shepard comes is the Citadel, where the Illusive Man, rebuilt with Reaper technology, takes control of Shepard's and Anderson's bodies. That's direct physical control, not the slow kind, and if the player can make the Paragon choice, Shepard talks him into seeing that he's the one who has been indoctrinated. (Mass Effect 3)

Our reading, and only a reading: Shepard never stays still long enough. Every Reaper encounter in the trilogy ends with Shepard leaving, winning or destroying the thing. No line in any game claims Shepard has a stronger will than anyone else. (Mass Effect; Mass Effect 2: Arrival; Mass Effect 3)

The Indoctrination Theory (a fan theory)#

After Mass Effect 3 came out in March 2012, a popular fan theory argued the opposite: that Shepard was being indoctrinated all along, and the ending is the Reapers' final push. Everything after Harbinger's beam hits Shepard in London, the theory says, happens in Shepard's head. Fans point to things the game really does show. Shepard has recurring nightmares about the boy seen dying on Earth in the opening, and the Catalyst appears in that same boy's form. If the war effort is high enough and the player picks Destroy, a final shot shows Shepard taking a breath in rubble. (Mass Effect 3)

In the theory, Control and Synthesis are Shepard giving in, Destroy is Shepard rejecting the Reapers, and the breath is Shepard waking up on Earth. It's an interpretation built from those details. The game never confirms it. (Mass Effect 3)

What the Extended Cut says#

BioWare's answer to the backlash was the Extended Cut, a free update released on 26 June 2012. Before it came out, BioWare's blog said it would add scenes and epilogues to the existing endings, that it wasn't a new ending, and that no further ending DLC was planned. (BioWare Blog, Mass Effect 3 Extended Cut FAQ)

What it added treats the endings as real. A new scene shows Shepard's squad either cut down by Harbinger or wounded and picked up by the Normandy before the final run, which explains how they end up on the ship. The Catalyst gets more dialogue, a fourth choice lets Shepard refuse, and each choice gets an epilogue showing what happens to the galaxy afterwards. None of that fits neatly with "it was all a dream", although some fans still read it that way. BioWare never endorsed the theory. The page on what happened to Shepard after Mass Effect 3 covers each ending's outcome. (Mass Effect 3: Extended Cut)

Read more: indoctrination in full, and the Mass Effect 3 ending explained. (Mass Effect 3)