(http://www.studymode.com/5th-wave-notes/plot-summary/)

The 5th Wave takes place in an alternative present-day reality in which an alien race known only as “the Others” is systematically annihilating humanity in order to inhabit Earth themselves. The central protagonist is sixteen-year-old Cassie Sullivan, who is on a mission is to rescue her five-year-old brother, Sammy, from a military compound run by the Others. This compound, called Camp Haven, is one of many locations where the Others–who look and sound exactly like humans, rendering them impossible to identify–are putting the 5th Wave of their annihilation plan into action. At Camp Haven, young humans are brainwashed into believing that the Others are human and that their fellow humans are Others. In this way, an army of traumatized, isolated children is being trained as the instrument of humanity’s ultimate destruction.

As the novel opens, Cassie is hiding out alone in the woods outside of Cincinatti, Ohio. She has been on her own ever since her young brother was taken from Camp Ashpit, a refugee camp where a large number of plague-infected human bodies were burned. After Sammy was moved to Camp Haven, Cassie’s father was shot dead by Colonel Vosch, the alien leader who appeared to be a trustworthy member of the U.S. military. After Mr. Sullivan’s murder, Cassie witnesses Colonel Vosch placing a bomb referred to as “the Eye” in the middle of Camp Ashpit and blowing it to smithereens. Cassie has been hiding in the woods ever since, unable to figure out how to rescue her brother but determined to do so. She tells us that humanity is now in the 4th Wave of the alien destruction, which is characterized by stealthy assassins who are indistinguishable from humans. Because the Others look and sound human, Cassie tells us that the first rule of the 4th Wave is: Trust no one. The second rule is: Stay alone if you want to stay alive.

In flashbacks, Cassie describes the first three waves of the alien invasion, which come in rapid succession. First, a strange mother ship hovers visibly near Earth for ten days but makes no attempt at communication. Many people, including Cassie’s father, optimistically believe that communication with the alien race will bring positive changes for humanity. This hope dies out when the aliens initiate the 1st Wave: an electromagnetic pulse that destroys all technology on the planet, rendering electronics, automobiles, and most forms of transportation and communication worthless. The 1st Wave kills half a million people. About three billion more are killed during the 2nd Wave, in which the aliens cause massive tsunamis to rise up across the Earth, decimating coastal populations. In the 3rd Wave, a viral plague carried by birds wipes out nearly everyone else. Only about 3% of the Earth’s remaining population–those with a natural immunity to the virus–manage to survive the 3rd Wave. Cassie, her brother, and her father all have the immunity. However, Mr. Sullivan does not survive the 4th Wave, the wave that Cassie calls “Silencer.” In the 4th Wave, the Others have finally come to Earth to do their killing in person. The only problem is that the Others are indistinguishable from humans, so it is impossible to trust anyone unless you already know him or her. Alien assassins pop up out of nowhere, simply shooting people dead where they stand. Cassie thinks of these assassins as “Silencers.” She carries with her an M16 and a Luger at all times.

As Cassie makes her way back toward Camp Haven to rescue her brother, she is shot in the knee by one of these Silencers. Cassie regains consciousness inside a clean, well-supplied Ohio farmhouse, where she is being cared for by a nineteen-year-old boy named Evan Walker. Cassie is immediately attracted to the handsome, kind young man, who says he lost his entire family and is living alone in the farmhouse, but she cannot quite bring herself to trust him. Clearly, Evan is in love with Cassie and, after she has recuperated, he insists on coming with her to rescue Sammy. Eventually, Evan admits to Cassie that he is really one of the Others, that he is in fact the Silencer who shot her, but that his love for her saved him from finishing the job. He explains that the Others implanted themselves within a certain number of human beings while the infants were still in their mothers’ wombs. The human hosts were completely unaware of the alien consciousness within them until “the Awakening”–which happened with the arrival of the mothership in the Earth’s atmosphere. Thus, Evan Walker is both human and Other. For Cassie’s sake, he is trying to renounce his alien side.

Meanwhile, the two-part narration of the novel also follows the experiences of Ben Parish, a charming high school athlete on whom Cassie has had a crush since third grade. After surviving the 3rd Wave, Ben has been indoctrinated by the Others and is training to be a soldier in their army. Because Ben has been successfully brainwashed, he believes that the Others are really humans. Led by Colonel Vosch, they appear to be a stronghold of U.S. military forces training the last surviving humans to fight the aliens. Cassie’s brother Sammy is assigned to Ben’s squad for training, and Ben develops an affection for the child. When Ben’s squad is deployed to fight, he promises Sammy that he will return for him. During their first mission, Ben realizes–with the help of Ringer, a talented and intelligent young woman soldier–that they have all been deceived. Ben and Ringer discover that the tracking chips implanted in their necks are also connected to a “kill switch” and that they could be executed at any moment. After removing these chips, they devise a plan to return to Camp Haven and rescue Sammy from the Others, whom they now realize are simply training humans to kill one another.

Ben’s and Cassie’s storylines collide at Camp Haven, which they both infiltrate for the same purpose: to rescue Sammy. With Evan’s help, Cassie, Ben, and Sammy all manage to escape from Camp Haven before it is completely destroyed. They are picked up by Ringer and the rest of Ben’s squad–all of whom now understand the truth about their military training–and the novel closes with the protagonists safe inside a Humvee. Evan Walker is not with them, but it is unclear whether or not he survived the destruction of Camp Haven. Thus ends the first part of Yancey’s trilogy.