All little girls eventually grow up, but that's not always the case for stories about little girls.

So often, series geared toward young adults fail to grow up with the audiences reading and watching them. But the gradual sunset that darkened the "Harry Potter" universe as time went on is a perfect example of series going darker as time goes on. (Harry went from an 11-year-old knocking out dumb trolls in his school bathroom to a young man fighting the biggest source of evil in his world.)

The latest movie in the "Hunger Games" universe, "The Hunger Games: Catching Fire," also nails this note, expanding its universe, upping its stakes and growing grimmer while still keeping its electricity.

Warning: "Catching Fire" and "Hunger Games" spoilers ahead.

It's only been a year and a half since audiences' first glimpse of "The Hunger Games," and "Catching Fire" picks up very shortly after the first film's conclusion, but much changed in Katniss and Peeta's worlds during their time in the arena, as we see on the Victory Tour.

It's not like the premise of the series was lighthearted to begin with — a battle to the death between two dozen impoverished and oppressed teens while government officials rub their hands together with glee and force the competitors' families to watch isn't the stuff of bedtime stories — but there's no arguing that grim goes grimmer.

The most obvious and brutal example of this is when the champions visit District 11, where Rue hailed from. After heartfelt speeches to Rue and Thresh's families, an elderly man whistles the four-note sequence that Rue and Katniss used to signal each other in the arena, then holds his hand up in District 12's three-finger salute. Peacekeepers immediately knock him over and drag him front and center while rushing Katniss and Peeta away; a gun is held to his head and the trigger is pulled just as the door slams behind them. Moms will be checking their phones worriedly — "Are you sure this movie is only PG-13?"

In another scene, Gale is on the receiving end of a brutal whipping. Katniss, Haymitch and Peeta all step between him and the peacekeeper delivering the blows, but the message is clear: this is not your grandmother's Panem. Gamemaker Plutchard Heavensbee impishly suggests broadcasting a series of floggings and executions to teach the populace a lesson about staying in their place.

It's not just the characters' bodies that are more clearly in danger in "Catching Fire"; their minds are as well. In the opening of the film, Katniss is hunting a deer in the woods. When she lets her arrow fly, however, the animal abruptly morphs into a man, felled. It's only after Gale arrives and calms her that the audience (and Katniss) realize that it was a sort of PTSD hallucination. In the same vein, the jabberjays in the arena call out with Prim's screaming voice, taunting Katniss and making her think her little sister is being tortured.

The whole attitude of the movie is darker as well. In the first movie, Katniss gorged herself on Capitol food and there was no social commentary, just cutesy remarks about its quality compared to District 12 fare. In "Catching Fire," we're introduced to a sort of high-class ipecac at a Capitol party, allowing revelers to barf up their food and make room for more. Peeta remarks bitterly that there are people in the districts starving, a line that never would have made it into the first film.

It's a credit to Francis Lawrence, who took over for "Hunger Games" director Gary Ross and will also direct the two "Mockingjay" movies that end the series. The tension in this movie is real, and the drama feels genuine. Suzanne Collins' series is a barely veiled sociopolitical commentary, and the first pass at the adaptation was a little surface, a little light. A little fist-bitingly "gosh, look at all these clothes!" The Katniss of the books is tough, and her world is by no means safe. In "Catching Fire," by going dark, the filmmakers are playing the game right.