The chapter people cite when they explain what this story is

It arrives before you have your footing

Fullmetal Alchemist opens in a register that reads, briefly, as adventure. Two brothers with extraordinary abilities, a quest with a clear object, a world with rules you can learn. The Tucker arc arrives before that register has had time to set, and it establishes — permanently, and without argument — that the rules do not include a guarantee that the small and the kind will be protected.

Placement is doing most of the work. Later losses in the series are heavier in raw terms; more people die, and people the reader has known longer. But those losses land on ground this arc prepared. A reader who has been through it does not relax again, and the story spends the next several hundred chapters cashing in that vigilance.

The motive is small, and that is the problem

Fullmetal Alchemist has villains with cosmological ambitions. It has a national conspiracy, immortality, mass sacrifice. Shou Tucker has a performance review.

He is not powerful, not charismatic, and not committed to anything larger than himself. He is a mediocre researcher who could not repeat his one result and did not want to lose his funding. Every element of his reasoning is legible to an ordinary adult, which is precisely why he is harder to file away than the genuine monsters. You cannot dismiss him as an aberration of nature. He is an aberration of incentive.

The rescue never comes

Edward and Alphonse are problem-solvers. Their entire characterisation to this point is competence under pressure: they arrive, they understand the situation faster than anyone else in it, and they fix it. Here they understand it perfectly and can do nothing at all.

Stories that raise this kind of horror usually discharge it. A cure is found, a reversal is discovered, a cost is paid and the loss is undone. This one refuses. There is no route back through a transmutation, the narrative does not invent one, and the only thing resembling relief arrives from a murderer who is not doing it as a favour.

That refusal is the series' actual thesis stated early and without decoration: equivalent exchange does not mean fair, and it does not mean recoverable. Everything the brothers later learn about their own bodies is foreshadowed here, in a case that has nothing to do with them.

The state signed off

It is easy to read this as a story about a uniquely repellent father, and easy to stop there. The text does not stop there.

The military certified him on the strength of a result nobody investigated. It funded him. It set the deadline that produced the second attempt. It sent examiners who would have accepted the new chimera and recorded another success, because the assessment framework asks what a specimen can do and never asks what it used to be. Tucker is the hand. The apparatus that only ever wanted to see the finished chimera is the rest of the argument — and the series returns to it repeatedly, as the military's use of human-derived chimeras turns out to be neither rare nor accidental.