Having discovered a dangerous fog, Katniss yells for Finnick, Mags, and Peeta to run away as fast as they can. The four run through the jungle. Katniss is fast, and she has a sudden urge to abandon Peeta. Even so, she hangs back and helps him through the thick foliage. Finnick doesn’t slow down, though he shouts for everyone to follow him. Katniss notices that the fog is affecting her nervous system as well as her skin, and she feels irrationally afraid.
During the course of the Hunger Games, Katniss is confronted with various moral dilemmas—she wants to kill Finnick in cold blood, she wants to abandon Peeta, etc. Ultimately, she doesn’t do either of these things, but she’s sorely tempted. The Hunger Games are a psychological struggle as well as a physical one.
The fog paralyzes Peeta’s legs, and Mags is similarly unable to move. Finnick volunteers to carry Peeta, and tells Katniss to carry Mags, who’s only about 70 pounds. Together, they move toward the water around the island. Katniss is grateful to Finnick for saving Peeta on multiple occasions. But before they get far, Katniss begins to feel her legs shake from the gas. She begs Finnick to carry Mags, but Finnick, crying, tells her that he can’t—his arms aren’t working. Mags silently crawls into the fog, and seems to die almost instantly.
Mags’ purpose in the plot of the novel seems to be that of a sacrificial lamb. Thus, she tastes the nuts when nobody else is brave enough, and she kills herself by walking into the fog in order to save the others from having to carry her. Mags’s death inspires more guilt from Katniss—it’s her own weakness, after all, that prevents her from carrying Mags, and thus condemns her to death. At the same time, Mags sacrifices herself, and there is strength in that.
Peeta, Katniss, and Finnick try to carry on, abandoning Mags’s body. Just when it seems that they’ll be unable to move any further, they notice that the fog seems to have stopped—it’s reached some kind of barrier. The group manages to crawl to the water. There, Katniss forces herself to submerge her blisters in the salt water, which is highly painful, but also eases the blistering. She sees that Finnick cannot move—he’s breathed in too much of the fog. She splashes salt water on his body while Peeta, who seems to have recovered somewhat, helps her.
In this section, we gain new sympathy for Finnick. He’s just lost someone he presumably cares about a great deal, and he’s also endured great pain as a result of his heroic decision to slow himself down and carry Peeta. From the perspective of a “fan” of the Hunger Games, Finnick is the most heroic character right now, since he’s risked his own life several times already for others’ sake.
Finnick slowly regains control of his muscles. Katniss notices that he looks weary, not only because of his running, but because he’s just seen Mags, one of his only friends at the Games, sacrifice her life. For the next few hours, the three slowly recover. They swim in the water and do simple muscle exercises. The fog’s effects seem to wear off.
The group has narrowly survived the encounter with the fog, even if they’ve lost a member along the way. It’s still not clear how the island “works,” but the group must decipher its secrets if they are to survive.
After recovering, the group ventures back into the jungle. There, they’re confronted with an intimidating sight: a huge pack of monkeys climbing through the trees. Peeta makes the mistake of making direct eye contact with a member of the pack, and this leads the monkeys to attack the group. Katniss uses her arrows to kill as many monkeys as possible, while Peeta and Finnick use their knife and trident, respectively. In the midst of the fight, anther tribute enters the jungle: it is a morphling addict from District 6. In her delirium, she walks toward a monkey, and it bites her.
For all his strength and leadership, it is Peeta who makes the crucial mistake of looking the monkeys in the eyes. So far, Peeta is hardly the capable, able-bodied competitor he was last year: if it weren’t for Katniss and Finnick, he’d be dead. Katniss is upholding her earlier decision, though, by fighting to keep Peeta safe.