Unlike his sister, Marcus is a very introverted character who is afraid of taking risks and has trouble keeping his head in difficult and dangerous situations. He can be overly cautious and is easily frightened. However, he's also emotionally intuitive, often serving as a middleman between his father and Regan in an effort to help them understand one another. Marcus is kind and gentle, but also the most visibly traumatized by the arrival of creatures.
When Marcus grows ill, his mother begins scavenging a local pharmacy for medication. He is then carried home by his father. As they're crossing the bridge, they discover that Beau has brought along with him a toy spaceship that makes a lot of noise. Lee sits Marcus on the ground and rushes to Beau, but he is too slow, and Beau is killed by one of the creature.
More than a year later, Marcus has long since recovered from his illness and his mother is pregnant. Marcus pretends to drive the broken-down truck sitting in the fields. He looks in the rearview mirror and sees his father approaching. Lee calls him over and sends him back home, where his mother prepares dinner. After dinner, Marcus and Regan play a game of Monopoly. In all the excitement, Marcus accidentally knocks over the oil lamp and causes a small fire that is quickly extinguished by their father. And fortunately, the incident doesn't seem to attract any creatures.
The next day, after a bit of homeschooling with his mother, Marcus goes hunting for fish with his father, though he is reluctant. Lee takes Marcus to the river, where he's installed homemade fish nets to catch food. Lee loses control of the fish, and it flips into the water. Marcus nearly takes off running in fear that the sound will bring the creatures, but Lee explains that the loud roars of the river contain the smaller sounds. Lee takes Marcus just beneath the waterfall, where he shouts loudly free of the worry of alerting the nearby creatures and promises Marcus that he's safe. Afterward, Marcus asks his father why he didn't allow Regan to come and if he blames her for Beau's death. Lee insists that he doesn't, but Marcus informs him that Regan blames herself. Lee explains that it was no one's fault. Marcus asks if his father still loves his sister, which he does. In that case, Marcus advises Lee to tell Regan that he loves her.
On their way home from catching fish, Marcus and Lee cross paths with an elderly man standing over his wife, who was killed by one of the creatures. Lee pleads with the man to keep quiet. However, the pain is too much to bear, and he lets out a loud scream. Lee picks up Marcus and takes cover behind a tree as a creature arrives and kills the man.
Upon their return home, the boys discover the red lights are on and rush to the farm. Lee hands Marcus a flashlight and tells him to launch the rocket in order to lure the creatures away from the house. Marcus runs into the field, where he sets off the firecrackers, the explosions providing a loud enough noise to distract the creatures. While walking back home, Marcus is startled by the sounds of the rustling leaves within the cornfields. He flees in the opposite direction, wandering aimlessly before knocking himself out from running into a tractor.
Marcus reunited with Regan in the fields, and the two make their way on top of a grain silo and set a fire to hopefully catch their father's attention, but they run out of lighter fluid and the fire dies. Still, Marcus is certain that their father will come for them, however, Regan believes that he's truly only coming for Marcus. As he attempts to convince Regan to stay put, Marcus falls into the grain-filled silo. One of the doors falls just a couple of inches away. Marcus begins to sink into the grains like quicksand, so Regan dives in to give him a hand, and they use the steel door as a resting platform. One of the creatures then jumps down into the silo, but it flees after hearing the whining feedback from Regan's hearing aid.
Marcus and Regan exit the silo and reunite with their father, who sends them to hide in the truck, as he fights off the creature with an axe. However, he is badly wounded, causing Marcus to call out to their father and gain the attention of the creature, who then proceeds to attack the truck. In his final moments, Lee pulls himself up and screams out loud to distract the creature from his kids. After the creature charges towards Lee and rips him apart, Marcus takes the truck out of park, and it rolls down the hill to the house, where they run into the arms of their mother.
The Abbotts head down into the basement. Evelyn sits Marcus in a corner with the baby as she and Regan fend off the creature. Regan then realizes that the high-pitched frequency from her hearing aid is the creatures' weakness. The pain is so excruciating after Regan places the boosted implant on a nearby microphone to magnify the feedback that the creature collapses. Painfully disoriented, the creature exposes the flesh beneath its armored head and renders itself vulnerable to a shotgun blast from Evelyn.
Marcus and Regan exit the basement with their mother and newborn baby brother. While she retrieves the air tank from the barn, Regan finds a speaker in the basement, which she plans to uses against the creatures. Regan then sets a fire on top of the silo in hopes of a response. A single fire peers over the mountain. And so, the Abbotts venture beyond the sand path to seek a new home with whoever lit the flame.
They follow the fire to a rail yard, where Marcus steps on a bear trap while running through the field and finds himself in unbearable pain, screaming at the top of his lungs even as Evelyn covers his mouth and pleads with him to be quiet. Evelyn opens the bear trap clamped onto Marcus' leg, and with Regan's help, he limps inside, where they are greeted by Emmett, who helps them into the basement and hides them inside a steel, soundproof bunker.
After the creature leaves, Emmett hands Evelyn a bottle of alcohol to pour on Marcus' wound. Regan then plugs a headphone set into the radio and places the headphones on Marcus while he rests. Suddenly, Marcus starts hearing "Beyond the Sea" playing on the radio, leading Regan to suspect that it's a message for them to cross the sea. However, Emmett insists that it's not. Later that night, Regan awakens Marcus from his sleep and leads him into the bunker, where she tells him about her plan to go beyond the sea as the song says in search of help or possibly a community. However, Marcus refuses to leave and attempts to convince Regan to not go through with her plan. However, when he awakens the next day, he finds a message on the radio that reads "Keep listening." He realizes that Regan is gone and tells his mother.
Evelyn lays the baby in the box and notices that the tank is just about out of air. So, she decides to take a trip to the pharmacy on her own. Marcus attempts to join her, but she convinces him to go back down into the tunnels and watch over the baby. Marcus lays the baby to rest and places him in the box, unaware that the tank is nearly out of air. He then heads up to the surface and looks around the factory. He finds where Emmett sleeps. And upon further examination, he finds the body of Nora, Emmett's wife. Marcus, startled, knocks over some equipment and causes a bit of noise enough to attract a Death Angel.
Marcus hides inside the bunker with the baby, as a Death Angel is now in the abandoned factory, hunting life. However, he accidentally locks himself inside the bunker. Running out of air inside the bunker, Marcus is forced to share air from the tank with his baby brother. His mother returns and after distracting the creature, Evelyn makes it into the basement, where she finds Marcus unconscious in the bunker, though he eventually awakens. She then shuts the bunker door without locking them inside until the creature leaves. Inside, they share limited oxygen. Sometime later, Evelyn exits the bunker but quickly realizes that the creature is still near and runs back inside. However, she is unable to get the door closed, and the creature reaches inside in an attempt to grab them, but it only manages to claw at Evelyn's leg. As the creature claws at them, Marcus hears the frequency coming from the radio and unplugs the headphones in order to fight back against the creature. Marcus forces it to cower at the sound of the high-pitched feedback from the radio until he can reach the gun sitting on the table and fire a single headshot to kill the creature.