Two exceptionally skilled operatives are assigned to guard towers located on opposite ends of a large, top-secret gorge, safeguarding the world from an unknown, sinister force that resides within. They form a connection from afar while remaining alert against an invisible adversary. As the catastrophic danger to humanity becomes apparent to them, they must collaborate to test their physical and mental capabilities in order to protect the secret hidden in the gorge before time runs out.
The isolated American sniper Levi, a former Marine, has no personal connections. He is recruited by the influential Bartholomew for a classified mission to oversee the West Tower of a gorge for one year, completely cut off from the outside world. He agrees to the task and is sedated to keep him unaware of the Tower's location. Upon arrival, he meets the previous guardian, J.D. from Britain, who informs him that his tenure has concluded, and Levi will now be in charge of monitoring the gorge and maintaining the equipment. Additionally, he learns that the gorge is concealed by various cloaking antennas, and there are sensors and mines along its walls to contain creatures known as "The Hollow Men." J.D. also shares that 2,400 soldiers vanished in the gorge during the 1940s.
He mentions that a siren will sound if the "Stray Dog" is activated, prompting Levi to evacuate the Tower and flee as far as possible. After bidding farewell, J.D. heads to the extraction point only to be killed by his rescuer instead of being rescued. Meanwhile, a woman arrives at the East Tower, catching Levi's attention. Eventually, Drasa, the woman, breaks protocol and reaches out to Levi through written messages, revealing it is her birthday, and they begin exchanging letters. One day, Levi sends a zip-line to her tower, visits her, and they share an intimate night together. However, as he returns to his tower the next morning, a large-scale attack by The Hollow Men occurs, causing his zip-line to fail and sending him into the gorge. Drasa quickly parachutes down to rescue him, and together they uncover the horrifying truth behind the gorge.