Summary
The story opens with a flashback of Cassie Sullivan's father as a child on a school field trip to a planetarium, where is inspired to name his future daughter Cassiopeia.
In the present, the remaining human population tries to avoid run ins with the "Others", the aliens that have wiped out most of humanity. A band of survivors hides in caves near Urbana, Ohio. One of them, a priest, turns out to be a Silencer. After killing the humans, the Silencer sits back to wait for the day the alien mothership drops the Final Wave, the green bombs that will complete the destruction of humanity and complete the Others' plans for Earth.
Ringer, enhanced with the 12th System by Vosch, returns to Vosch to achieve their mutual goal of killing the defected Silencer Evan Walker. Vosch gives Ringer a bomb contained in a pill so that she can kill herself instead of being killed by the Final Wave when the time comes. He dispatches Ringer and a Silencer named Constance to look for Evan in the caverns.
Cassie, Sam/Nugget, Ben/Zombie, and other members from Squad 53 are hiding at a safe house. They are later joined by Evan. Later, Zombie and Dumbo go to the caverns to find Teacup and Ringer, to warn them about the upcoming final Wave.
While in the woods, an attack ends with Dumbo shot in the back. Zombie hides him in a building in Urbana, and goes for help.
Ringer and Constance land near the caverns. Constance disappears and Ringer battles the Silencer priest, who is faster and stronger. Zombie also arrives at the caves, but falls into a large pit full of dead bodies and starts to sink. Constance returns, pulls him out, and shoots the priest dead.
Zombie, Ringer and Constance head into Urbana to reunite with Dumbo. Zombie is suspicious of Constance, but Ringer vouches for her, knowing that if she doesn't Constance will kill her friends. They return to Dumbo's hiding place, but Dumbo dies due to blood loss, and Zombie marks his grave with colored flags.
The three of them return to the safe house. When the trio arrive, Constance holds Nugget hostage, demanding Evan returns with her to Vosch. Nugget, using a gun hidden in his pants, shoots Constance. Her enhancements help her survive, so Ringer and Nugget shoot her repeatedly to finish her off. Ringer tells them that Vosch will see that Constance's implant is offline and will send a helicopter of 5th Wave troops to capture Evan and kill everyone else. Evan volunteers to return to Vosch.
As the helicopter approaches, everyone hides in the basement of the safe house. Ringer and Evan kill many of the attacking soldiers, revealed to be Ringer's original squad before she was transferred to Squad 53. Evan is captured and taken away in the helicopter. Ringer devises a plan for those in the safe house to attack the soldiers that will come to bring back the survivors from the first chopper. During this planning and waiting, Ringer tells Zombie that she is pregnant with Razor's baby - a 5th Wave soldier who helped Ringer escape from Vosch at the end of the previous book. When the next chopper arrives, they kill all the soldiers, capture the helicopter pilot, and leave a wounded Zombie behind to take Nugget and Megan to the caves. Ringer forces the pilot to fly them to Vosch's base.
At the base, Vosch finally explains the aliens' actions to Evan, that the Others have gone to many planets and attacked dominant species in order to save the lives of all other species before the dominant ones can destroy themselves and their planet's ecosystems. The 5th Wave will drop bombs on every city, removing all traces of mankind, resulting in humans remaining in a permanent stone. When this Final Wave succeeds, Vosch's reward will be a launch in an escape pod to the mothership to become an immortal consciousness.
Vosch examines Evan and discovers that the glitch in the 12th System is love, so he reprograms him to have all of his humanity erased, leaving him as an emotionless killing machine. He plans to have Evan kill Ringer when she arrives.
At the caves, a helicopter squad gets in a battle with Zombie, Nugget and Megan, resulting in Zombie being shot in the back of the head.
Ringer and Cassie infiltrate the base. Cassie, dressed in a soldier uniform, and Ringer split up. Ringer detonates a bomb, allowing Cassie to hide among the troops. The pair head to the control base, with additional bombs being set off. Cassie makes it to the door where she is to wait for Ringer, when General Order Four activates, a small version of the first three waves the Others used when they attacked Earth, with poison burning her skin from the sprinkler system. She leaps outside, trying to get the poison off her skin.
At the Urbana caves, Zombie comes to. A Sergeant asks where Ringer is, holding Nugget hostage. The squad take Nugget, Zombie and Megan back to the base.
Ringer finds Cassie, and swaps outfits with her, knowing the poison won't affect her. The pair descend into the base, entering the computer core and try to find Evan's. Cassie decides to download all 10,000 people's worth of memories into her mind, learning all of the access codes and the layout of the base.
Ringer finds Evan in the hall, and they fight. Evan is much faster, stronger, and more ruthless than Ringer, breaking her back. Ringer screams to Cassie to run. When Evan enters the room, he only finds bits of Cassie's hair and blood in the chair, as she has torn herself out. As he uses his super hearing to find her, entering a flooded room, where Cassie drops a live electrical wire into the water, electrocuting him.
Vosch finds Ringer, still paralyzed. He explains that he thinks of himself as Ringer's creator/father, and wants to take her with him to the mothership. She turns him down, distracting him until Cassie appears and shoots him in the head. Cassie emotionlessly tells Ringer that she killed Evan, and Ringer consoles her, stating that he was already gone, having been turned into a shell of a person without humanity.
Cassie, uses a keypad to open a secret door to Vosch's escape pod. Cassie leaves in the pod and Ringer searches her pockets for the explosive pill Vosch gave, so she can wipe out the base. She remembers that she switched outfits with Cassie, so Ringer drags herself to Vosch's body and searches his pockets for the kill-switch that will kill all 5,000 soldiers at the base who have tracker implants.
Zombie, Nugget and Megan are in the helicopter, which has been circling the base. The soldiers suddenly stiffen and die, resulting in the trio jumping into a river to survive the crash. As they head to the base, they see a green light launch and head towards the mothership.
Cassie finds the explosive pill in her pocket and recognizes its similar to the one Megan had, which is activated by the carbon dioxide in a person's exhaled breath. The escape pod docks on the mothership, which is filled with the high explosives that are to be dropped on Earth's cities. She recites her and Sammy's night-time prayers one last time before sacrificing herself to destroy the ship. Zombie witnesses a large explosion from the mothership which then disappears. He finds Ringer paralyzed in the hallway and realizes that Cassie has sacrificed herself.
Several months later, Zombie and Nugget are in a toy store in Texas. They come across a soldier with one hand in his shirt holding a wound—similar to a scenario Cassie experienced in the first book. Nugget wants to kill the soldier and Zombie tells him that they don't do that anymore. Zombie tries to reason with the soldier, but the soldier pulls a gun and shoots himself in the head. Zombie and Nugget return to a house where Ringer is nursing her newborn baby girl, named Cassie. Outside, Evan Walker is patrolling – the 12th System saved him from the electrocution, and his human friends have re-downloaded his memories, returning his humanity. Evan tells Zombie that he plans to go to as many 5th Wave military bases as he can and destroy them, calculating that for every life he takes, two will be saved. As Evan leaves, Nugget watches from the house. He asks Zombie about the constellation that was his sister's namesake, and Zombie points out that three of the stars are a throne that the constellation Cassiopeia sits on, from which Cassie now sits to watch over the realm.
