The main character, a young, unnamed boy, comes across the spider in a dark forest, where the spider attempts to kill him with its sharp legs. Representing a giant arachnid, the creature continually follows the Boy throughout the Forest. It is an opportunistic killer, impaling the boy whenever one of its eight legs is within range.
When the boy first meets the Spider, it tries impaling him, but the Boy uses a bear trap to cut off three of its legs. Enraged, the Spider goes on a rampage throughout the forest, killing other children in order to get to him. Not long after, the boy gets trapped in the spider's web inside a tunnel. Catching the boy, the spider wraps him in webbing and hangs him from the ceiling. Fortunately, the boy manages to wiggle free, but with the spider in hot pursuit.
After evading the arachnid through great chasms and small caves, he releases a boulder onto it which knocks the spider into a small hole, sending it plummeting into a pit. However, the spider again returns, this time with only one leg remaining. The Boy then pulls the one leg off after dodging an attack, and uses the body as a bridge to get across a chasm.
- The Spider's motives for attempting to kill the boy are unknown; some argue that the spider is simply hungry, or that the Spider represents Death (or the Grim Reaper). One particular theory states that when the Boy was alive, he suffered from arachnophobia (the fear of spiders.) The arachnid could be the manifestation of his fear.
- The spider appears to be notorious in the world of Limbo, and the other children hide from it and fear it. They set traps and other defenses to protect themselves from it. They even try to imitate it by building a machine similar to its impaling legs.
- There is a glitch that can occur when the spider has one leg left. When the leg is about to stab the boy, the boy can run under the leg and not be stabbed. However, this causes the boy to be knocked onto his back and die.
- For a strange reason, the boy dies whenever he falls on his back or face, except for when it is supposed to, such as when he falls down a hill from escaping the spider.