The Stanford Prison Experiment is about, surprise! the Stanford Prison Experiment. Where a professor in the 1970s paid some student volunteers to be prisoners and others to be guards, the idea being to observe their behavior over a 2 week period. Except it was called off after 6 days because things had gotten entirely out of control. The "guards" were abusing their power to an absurd and disturbing degree. The "prisoners" were begging to be released, but were being confronted with the professor and his assistants, who adopted the role of being an unsympathetic parole board. So they browbeat them, accuse them of slacking or trying to shirk their punishment, promise them one thing, then refuse to honor it. And even though the contract the students signed states they can leave at any time, they are repeatedly blocked from doing so, only being allowed it when it really looks as though they're going to snap entirely.
So it's a depressing, infuriating viewing experience. Because it isn't people with power or authority hurting others through indifference or ignorance of the damage they're doing. It's people deliberately choosing to hurt others. One of the guards at one point tries to heckle a "prisoner" about how much he probably misses his home. The "prisoner" admits he doesn't have one. He can't afford an apartment, so he'd been living in his car. By volunteering for this, he gets a bed (or cot, at least), some food, some money. The guard is temporarily stymied by this, but by the following night, he's decided to try even harder to humiliate and abuse this guy, who is being totally obedient. Because he can, basically. It amuses him to do it, or maybe it infuriates him that this guy isn't being broken and degraded. They show some footage of interviews with the volunteers after, and that guard tries to claim he was doing his own experiment about how vicious he could get before someone spoke out, but it reeks of him trying to justify his actions after the fact. Or he's a sociopath. Whichever.
I don't know how historically accurate the film is. I assume there are some things that were exaggerated for dramatic effect, but I don't know which parts. The part where the lead professor is sitting in the hall keeping watch, because there's a rumor the first "prisoner" they released from the experiment is going to come to spring a jailbreak, meets another faculty member, who innocently asks what the independent variable is in all this, and the professor can only bluster because there isn't one, that felt a little fake. A little too much of the obvious moment where the supposedly noble scientist has a chance to realize this can't prove what he wants, but pushes ahead anyway out of ego. But maybe it actually happened that way.