Their vocal sound effects were provided by Justin M. Davey.
The Monsters are emaciated, vaguely humanoid creatures whose entire bodies are covered by many plates of chitinous armor. They are stated to be "evolutionary perfect" machines that evolved to withstand firearms and even the destruction of their own planet. They also appear to have arms with prehensile digits, but they are only ever observed using their limbs for locomotion. They have abnormally sharp claws that are strong enough to cut through trucks and metal, and is their main weapon for killing too. They are also inhumanly fast, able to quickly move to locations faster than their prey can react.
Due to their lack of eyes, the Monsters hunt by sound, and are completely blind. They have sensitive plates lining the front of their faces that can extend and contract to help them gauge direction of where sound is coming from. The back of their heads are capable of unfolding into massive ear-like structures with pronounced, slimy eardrums. Using these, they can detect even the most subtle of changes in sounds as little as a floorboard creaking miles away.
A notable ability the Monsters have is the ability to affect electricity. As seen multiple times in the film, electronics like lights tend to flicker in a Monster's presence. Why and how they do this is completely unknown.
Although the origins of the Monsters are largely unknown, they existed for nearly a millennia before an asteroid carried them to Earth. Afterward, the destructive asteroid crash-landed in Mexico with the power of a nuke, where they began spreading across the nation and eventually the entire world. They killed anything and everything that made noise, assisted by an incredibly powerful sense of hearing far beyond anything produced on the Earth. This combined with their impenetrable armor, allowing them to fight off any resistance, including Earth's militaries, and all of the sapient life on the entire planet, which subsequently caused society to crumble and collapse.
Over the span of three months, most of Earth's human and animal populations have been wiped out by the Monsters with the US military (and presumably other armies as well) giving up on trying to fight back against the them. The Abbotts family — the wife named Evelyn, the husband named Lee, congenitally deaf daughter named Regan, and the two sons named Marcus and Beau — silently searching for supplies in a deserted town. While out in the open, the family communicates with American Sign Language (ASL). Four-year-old Beau is drawn to a battery-operated space shuttle toy, but his father takes it away. Regan returns the toy to Beau, who also takes the batteries his father removed. Beau activates the shuttle when the family is walking home through the woods, near a bridge. Its noise makes him an instant target for a nearby creature and he is swiftly killed.
Later on, as one of the children knock over a lantern, the family prepares for a potential monster attack as something lands on the roof. However, it only turned out to be some raccoons that jump off the roof and can be seen from the window. Then, one of the monsters pounces on one of the raccoons after it moves away from the house, crushing it in one swift stroke.
While fishing with his son, Lee encounters an old man in the forest whose wife was killed by the Monster. Although Lee urges the man to stay quiet, the old man was driven to suicide. He screams to draw the Monster to him, not even caring that he had placed an innocent man and his child in danger by doing so. A Monster then arrives and kills him in one swift motion, unaware that Lee and his son were nearby.
Alone at the house, Evelyn goes into labor earlier than expected. While making her way to their house basement, she steps on a large nail. In pain, she accidentally drops a glass picture frame and alerts a nearby creature. Evelyn flips a switch that changes the exterior house lights to red as a danger signal to the others, and struggles to remain silent during her contractions. Arriving at the farm and seeing the lights, Lee instructs Marcus to set off fireworks as a diversion. He finds Evelyn hiding in the bathroom with their newborn son. The baby starts crying, attracting a creature. And they narrowly avoid another attack on the way to their improvised soundproofed barn basement. Lee leaves to find the other children, after promising Evelyn he will protect them. Evelyn falls asleep, but wakes to discover that the barn basement is flooded with water from a broken pipe and that the creature is stalking her.
Regan hurries back to the farm. She and Marcus take refuge atop a grain silo, lighting a fire to alert their father to their whereabouts. However, they run out of lighter fluid and the fire dies before they can attract Lee's attention. A hatch door gives way and Marcus falls into the silo. The sound of the door falling distracts the creature stalking Evelyn which then runs towards the silo. Marcus sinks into the corn and nearly suffocates, but Regan jumps in and saves him. They avoid further danger by clinging to the fallen silo door and survive a subsequent creature attack by hiding under it. Regan's cochlear implant reacts to the proximity of the creature by emitting a piercingly loud high-frequency sound that drives the creature away. The children escape from the silo and reunite with Lee.
The creature returns, attacks and wounds Lee, while Marcus and Regan hide in a pickup truck. After seeing his father wounded, Marcus shouts impulsively, attracting the creature to the truck. Due to its unpleasant sound, Regan turns her cochlear implant off for the first time, unaware of the fact that it would have potentially driven the creature away. Lee uses sign language to tell Regan and Marcus that he has always loved them before sacrificing himself by yelling to draw the creature away from his children, so that way they can escape. Regan and Marcus roll the truck down a hill to escape and reunite with Evelyn and the baby at the farmhouse.
The four retreat to the house's basement, which to this point was off-limits to the children. There, Regan sees her father's notes on the creatures and his experimentation with several different implants. When the creature returns to invade the basement, Regan realizes that the sound made by the implant distresses the creature and hence she switches the device back on and places it on a nearby microphone, magnifying the feedback to ward off the creature. Painfully disoriented, the creature exposes the flesh beneath its armored head, which Evelyn shoots, killing it. The family views a CCTV monitor, showing two creatures attracted by the noise of the shot. Now armed with the knowledge of the Monsters' weakness and a shot gun, the family arm themselves, ready to fight back.
Because of their superb hearing abilities, high frequencies will disorient and irritate them and can be used to immobilize them or even drive them away. Extremely high frequencies can and will kill them. While the outside of their body is heavily armored, their insides are soft and not protected by armor, and a well-timed shotgun blast to a Monster's sensory organs or mouth can instantly kill it. However, doing so risks attracting the attention of even more Monsters. The best possible method for survival is to remain as silent as possible.
The Monsters are not particularly smart either, as they are shown to have predatory intelligence and exclusively attack things that make sound, such as when the Monster was attacking the vehicle Regan and Marcus were in instead of attacking them. Due to them being completely blind, actually finding a silent target is very difficult for them. Being near naturally loud areas, such as near rivers, will help drown out other sounds. They also can be easily distracted by other noises in the area, immediately forgetting about their target and running off to go find the other noise.
List of Victims
- Beau Abbott: Mutilated.
- Raccoon: Squashed to death.
- Old Man: Slashed across the chest.
- Lee Abbott: Mutilated.
- Old Woman: Stomach sliced open.
- John Krasinski, the director of the film, mo-capped the monsters in certain scenes.
- The Monsters are referred to as "Death Angels" and a number of similar ones in newspaper clippings appearing in various promotional material for A Quiet Place. In the movie itself, a clipping calling them "Dark Angels" appears, but they are never called by this name or any other.