The film begins matter-of-factly in an almost ominous way by showing us the typing and printing of the flyer advertising a need for students willing to participate in the experiment, and from there we meet Dr. Zimbardo (Billy Crudup) and his staff that will observe the experiment, as well as brief interviews with each of the key students involved. Once in the experiment, the situation quickly becomes uneasy for the 'prisoners,' and before long, degradation and violence ensues and threatens to compromise both the morality and validity of the experiment and the permanent mental health of the students involved.
The film is an interesting one for a couple of reasons: on the surface, of course, is the fascinating nonfiction material that it is based on. Even knowing what's coming, the results of the experiment are legitimately shocking and disturbing, and will almost certainly make any viewer think long and hard about both the prison system and the terrifying dangers of unchecked authority.
Aside from that is the somewhat unique and subtle storytelling methods that the film uses. While it appears to be simply telling the story of the real life event, it is extremely carefully written and edited so as to stay aloof, keeping the viewer at a distance. This works very well in that it makes you complicit in the experiment, a silent observer to both the students in the mock prison and the faculty running the show, which furthers the idea that they become part of the experiment themselves as they decide the fates of these students as they keep letting things get farther and farther out of control. This technique also doesn't allow you to empathize too much with the debased prisoners or to completely despise the guards, as the film's construction encourages you to stay neutral as you observe, as if your taking a side may somehow taint the results.
This all works very well up until the last 20 minutes or so, where the film has some issues. The careful perspective is somewhat abandoned here as we see more of the personal life of Zimbardo as his girlfriend (Olivia Thirlby) becomes horrified by his inability to remove himself from his warden persona, which also throws the pacing off a bit. This is further tampered with when the experiment is over, and the final few scenes are a bit strange. It's as if they weren't quite sure which ending would be the most impactful, so instead they use them all: there's Zimbardo speaking about the results; intertitles that explain the aftermath; and a conversation between the most vicious guard and the most terrified prisoner that seems as if it is reenacted word for word from the real thing (why not include that footage or audio?).
I would be remiss if I didn't mention some of the stellar performances, in particular by some of the younger cast: Tye Sheridan (Mud) and Johnny Simmons stand out as the emotionally damaged prisoners, but the real star is Michael Angarano as the brutal guard who decides to see how far he can push himself before someone steps in and stops him.
Despite getting slightly derailed in the end, the film is exceptionally intense and engrossing–its two hours fly by as you watch with disturbing detachment how quickly unchecked power can turn normal people into monsters. The source material alone is absolutely absorbing–you may find yourself poring through the endless records kept at the excellent official site. It is, however, only the latest in the event's myriad adaptations across media as diverse as manga and Veronica Mars, but it is noteworthy for its intensity and ambition.