Many years have passed since the human civilization was destroyed by their own creation, AI controlled war machines that could build themselves faster then military forces could destroy them. Leaders of the conflict saw there's no way of winning this battle, only counting down days until humanity is no more. So they've built a safeguard... something that could prevent humans from going completely extinct. Many decades later, animal-modeled machines roam the Earth and human civilization is starting to form from the very basic concepts... although with all kinds of technology lying around, not only primitive weapons are being used.

Tribes and cities have been built and established, new religions created, and as human kind is more bent on destroying than peacefully resolving things, conflicts and wars ensued. But not against the machines, but among the tribes. The player controls Aloy, a young girl who was shunned by her own tribe and got cast out to live alone in the wilderness. Luckily, she had another outcast to help her survive and teach her the way or the world, Rost, who acted as her father figure. In order to prove herself and rise above the hatred towards her that she herself did not understand true reason for, she put every effort she had into the hard work of making herself a great hunter and a formidable opponent.

Years have passed and she became an adult by the time she finally got semi-accepted by the tribe she got cast out from. But during her test as a hunter and a young warrior, an enemy force with heavy firepower attacked the tribe and in the conflict that ensued, Rost died protecting Aloy. Who were they? Where did they get such weapons? How could they control some of the machines? Aloy was no longer interested in becoming accepted by the society... she had one goal in her mind... to catch Rost's killers and make them pay with their blood. The only thing keeping her is the vast world separating them... and she was about to set out of her comfort zone and make a name for herself. And, in the process of doing so, find out she is more than just another tribe girl... she is the key to saving the human race, and defeating the AI.

Horizon is an action game with role-playing elements that mainly focus on spending points to upgrade character skills or enhance the weapons and items Aloy is carrying. The open world has several different regions from sandy dunes and lush forests to snowy mountains and open meadows. Aloy's different outfits provide better protection based on a certain terrain type of weather climate. When she is not fighting enemies, Aloy is a hunter and is more than capable of hunting different types of machines, or go on a stroll and pick flowers and herbs for brewing potions. Destroying machines earns experience points as well as parts which can be used to upgrade weapons or build useful items. Some machines can also be hacked if sneaked upon so Aloy can control them.

While bandit camps can be permanenty cleared, regions where machines roam can only be temporarily cleared until the player leaves the area. Upon return the machines are re-spawned so making roads safe is not an option, but on the other hand, gathering necessary parts for building items is always available. Aloy uses bow and arrow for ranged attacks and spear for close-combat as primary weapons, with some other more specialized and powerful weapons that are available but optional. World map shows regions that Aloy has visited and fast travel checkpoints also become available after a while. A faster way to show more of the map is to track down giant walking machines, climb on top of them and hack them. This will reveal a huge region around the player as well as areas of interest. As is the case with open world games, the game story progresses by completing main objectives, while leaving many different types of side quests and optional but available.

The game also includes a detailed photo mode which lets the player create and capture screenshots that are not achievable with in-game graphics filters and camera angles.