In the "Solo Leveling" anime's first season, Sung Jinwoo, a weak E-rank hunter, gains the ability to level up through a mysterious system after surviving a dangerous double dungeon, becoming the solo player and embarking on a journey to discover the secrets of the dungeons and his powers.
Sung Jin-woo, considered the weakest Hunter in the world, enters a dungeon gate with his friend Lee Joo-hee and other stronger Hunters. After the boss is killed, the gate does not collapse but instead reveals a second connected dungeon. The party is split evenly on whether or not to proceed; Jin-woo casts the tiebreaker to send them down into the second half of the double dungeon. The party finds the dungeon empty save for a large room filled with statues, which come alive and begin slaughtering the Hunters.
Jin-woo realizes that the dungeon they are in has specific rules by which to survive the statues - first, to "revere God," second to "praise God," and finally to "prove your faith to God." Several Hunters are killed as Jin-woo and the senior Hunter leading the party, Song Chi-yul, slowly manage to decipher the instructions on how to survive. Finally, only Chi-yul, Joo-hee (who has collapsed from the stress), and Jin-woo are left, and Jin-woo tells Chi-yul to evacuate with Joo-hee while he holds the door. Left alone, Jin-woo rages against his imminent death. However, as he dies, he receives a message that he has completed the hidden quest and may become a [Player] if he accepts the offer.
Jin-woo successfully defeats the boss, much to Jin-ho's shock. When Dongsuk and his group re-enter the boss room, he attempts to coerce Jin-ho into killing Jin-woo, but Jin-ho refuses. Jin-woo receives a new quest to kill Dongsuk's group under penalty of death, leading him to massacre them. He exits the dungeon with Jin-ho, using the cover story of the others having been slain by the dungeon monsters.
Jin-ho offers Jin-woo a building in exchange for his help in fulfilling requirements for guild certification; Jin-woo rejects the offer. Jin-woo notices his daily mission keeps incrementing past the requirements. Upon maxing it out, he is rewarded with another instance key - this time to an S-rank dungeon that rewards a special potion that can heal any illness; Jin-woo realizes it can cure his mother and decides to risk it. The first battle within the dungeon is against a guardian Cerberus, which nearly kills Jin-woo multiple times before he finally manages to slay the beast. His reward is the key to the castle Cerberus was guarding, within which is the ingredients he needs to craft the potion. Knowing he would die if he pressed on, he promises himself to return later and succeed. Meanwhile, the Hunters Association investigates the death of Dongsuk after learning the dungeon boss was defeated. Dongsuk's younger brother, the S-rank Dongsoo, blames Jin-woo and Jin-ho and swears revenge.
Jin-woo visits his comatose mother and decides to push forward with getting stronger so that he can get the needed ingredients to save her; he thus accepts Jin-ho's request to aid in running dungeons to allow creation of a guild. To fulfill the party member requirement for C-rank dungeons, the Hunters Association recruits the survivors of the double dungeon raid (Jin-woo, Joo-hee, Chi-yul, Kim Sangshik, and Kang Jeongho), and due to a shortage of Hunters in the area, the group is rounded out by several imprisoned Hunters, who serve in exchange for reduced sentences, that are overseen by B-rank Kang Taeshik.
At a three-way junction, the group splits up. Kang uses the separation to murder the prisoners at the behest of their victim's father, and when Jeongho and Sangshik's path leads them to stumble upon the scene, Kang kills them to cover it up. Jin-woo, Joo-hee, and Chi-yul race back to the scene and confront Kang, and Jin-woo reveals his newfound power to kill Kang. A grateful Chi-yul covers for Jin-woo. Later that night, Joo-hee asks Jin-woo if he "remembers", holding up a small Essence Stone.
Jin-woo starts the job-change quest, and is teleported to a massive castle. After fighting through waves of knights, assassins, and mages (apparently mimicking those he's fought previously) he confronts Igris, the Bloodred Knight Commander. Igris is every bit as powerful and moreso than Jin-woo, who only barely manages to slay the Knight. The actual job-change quest activates, unleashing an unending horde of the monsters he'd fought on the way to Igris' chamber. Meanwhile, the S-rank hunters of Korea gather and head for Jeju Island, years ago the site of an S-rank dungeon break.
Overwhelmed by the seemingly endless horde and recalling his prior circumstances, Jin-woo nearly despairs before deciding to press forward. The timer for the daily quest expires and he is transferred to a penalty zone, which he uses to recover and prepare himself. Returned to the castle, he succeeds in defeating first the horde. As a reward he gains the title "Shadow Monarch" and the ability to bring the dead, including Igris, back to life as his "shadow army." Meanwhile, the S-rank hunters make a horrifying discovery - the Jeju Island S-rank monsters have begun to evolve and threaten to spread to the mainland.