Hundreds of cash-strapped players accept a strange invitation to compete in children's games—with high stakes. But, a tempting prize awaits the victor.
Red Light, Green Light
Seong Gi-hun is down on his luck, having accumulated enormous debts with loan sharks while becoming estranged from his daughter and ex-wife. At a subway, a well-dressed man asks him to play a game of ddakji for money, and offers an opportunity to play more games with much higher stakes. Gi-hun accepts, is knocked unconscious, and awakens in a dormitory with 455 others, identified only by numbers on their tracksuits. A group of masked guards in pink jumpsuits arrive and explain that the players are all in dire financial straits, but will be given billions of won in prize money if they can win six games over six days. The games are overseen by the Front Man, who is masked and dressed in black. Gi-hun befriends Player 001, an elderly man suffering from a brain tumor. He also recognizes two other players: Cho Sang-woo, a childhood classmate who became an investment broker, and Player 067, a pickpocket who stole Gi-hun's money after he won a horse race wager. The first game is Red Light, Green Light, where anyone caught moving is shot dead on the spot, revealing the sadistic nature of the games; half the crowd panics upon realizing this, turning the game into a massacre. With help from Sang-woo and Player 199, Gi-hun finishes the game alive.
With over half of the players killed in the first game, many survivors demand to be released. Using the game's third clause, they successfully vote to cancel the game and send everyone home, but without any prize money. Back in Seoul, Gi-hun goes to the police, but no one believes him except Detective Hwang Jun-ho, whose brother received the same invitation card and has recently disappeared. The players are invited to re-enter the game, and many return out of desperation. This includes Gi-hun, whose mother needs surgery; Sang-woo, who is about to be arrested for financial fraud; Player 001, who does not wish to die in the outside world; Player 067, who wishes to rescue her parents from North Korea and get her little brother out of an orphanage; Player 199, a Pakistani migrant worker who attacked and gravely injured his boss for withholding his wages; and Player 101, named Jang Deok-su, a gangster on the run from gambling debts. Jun-ho secretly follows Gi-hun when he is picked up by the guards.
The Man With the Umbrella
Jun-ho successfully infiltrates the games by disguising himself as a masked worker, and it is revealed that the location is a remote island. The players are now more prepared and start forming alliances. Gi-hun, Sang-woo, Player 001, and Player 199 team up. Player 067 explores an air vent and witnesses workers melting pots of sugar. The second game is revealed to be Ppopgi, where each player must perfectly extract a stamped shape from a dalgona (honeycomb candy) under a 10-minute time limit. Sang-woo learns of Player 067's discovery and recognizes the game beforehand, but does not warn his teammates and chooses the simplest shape for himself. Gi-hun ends up picking the most difficult shape, an umbrella, but is able to complete the game by licking the back of the honeycomb to melt it. Player 212, a rowdy and manipulative woman, helps Deok-su complete the game with a smuggled lighter. Player 119, who is about to be executed takes a guard member hostage and forces him to unmask. Shocked that the staff member is a young man, the player shoots himself, and the guard is killed by the Front Man for revealing his identity.
Stick to the Team
Player 111, a disgraced doctor, secretly works with a handful of guards to harvest organs from dead players to sell on the black market, in return for information on upcoming games. When Deok-su kills a player accusing him of taking extra food, the guards do nothing to stop him, and the prize money's value is increased. After lights out, a riot erupts in the dormitory as players attack each other. Gi-hun's group survives and exchange names to build trust: Player 199 is Ali Abdul, and Player 067 is Kang Sae-byeok. Player 001, because of his brain tumor, has trouble remembering his name. Player 212, named Han Mi-nyeo, has sex with Deok-su. In the third game, players are told to form groups of ten. For Gi-hun's team, Sae-byeok recruits Player 240, a girl close to her age. The game is revealed to be tug of war on two raised platforms, where a team wins by dragging the opposing team off the platform to their deaths. Deok-su, having learned of the game from Player 111 beforehand, picks only strong men and rejects Mi-nyeo, who joins Gi-hun's team. After Deok-su's team wins their match, Gi-hun's team struggles against another all-male team.
A Fair World
Gi-hun's team wins their tug-of-war match with Player 001's strategy and Sang-woo's quick thinking. Anticipating another riot, they build a barricade and spend the night taking turns on guard, but Deok-su's team does not attack. Gi-hun reminisces on a similar situation from ten years ago, when he and many other workers from an automobile factory protested a mass lay-off, which caused the failed trajectory of Gi-hun's life. Player 240 attempts to befriend Sae-Byeok, but she doesn't reciprocate. Jun-ho witnesses the organ harvesting racket, as the guard whose identity he stole took part in it. Except Jun-ho, everyone involved in the racket is eventually killed, including Player 111. The Front Man begins a facility-wide manhunt for Jun-ho, who breaks into the Front Man's office. Jun-ho learns that the game has been running for over 30 years, and that his elder brother Hwang In-ho was the winner in 2015.
The players see the bodies of Player 111 and his co-conspirators strung up for cheating, and are assured that the games are designed to give every player a fair chance without discrimination. For the fourth game, players are told to pair up, but discover that instead of working as a team, they will have to play against their partner in a marble game of their choice. Whoever gets all their partner's marbles within 30 minutes will win and survive. Sae-byeok and Player 240, named Ji-yeong, share their life stories; Ji-yeong sees Sae-byeok has more to live for and sacrifices herself. Sang-woo tricks Ali into giving up his marbles and wins, which results in Ali's death. Deok-su wins against his partner and henchman, Player 278. Gi-hun exploits Player 001's dementia to defeat him, only to discover that Player 001 had been aware of the deception the entire time. Player 001, who remembers his name to be Oh Il-nam, allows Gi-hun to win anyway, as he is his gganbu (trusted friend). Gi-hun and Sae-byeok are traumatized by the deaths of their friends.
The players return to find Mi-nyeo, who did not have a partner for the marbles game, still alive. Foreign VIPs, who had been wagering on the games remotely, arrive to watch and wager on the next rounds live. Jun-ho, posing as one of the masked servants, is sexually propositioned by one of them. In a private room, he attacks the VIP, records his confession, and escapes the island. Meanwhile, the fifth game has players crossing a two-panel wide bridge, where the panels are each made of either tempered or regular glass, the latter of which cannot support their weight. The players at the front of the line fall to their deaths as they progressively test the panels. Deok-su refuses to move despite the clock ticking, daring others to pass him. Mi-nyeo grabs him and, as revenge for betraying her, pulls him down with her to their deaths. Player 017, a seasoned glass worker, is able to determine which panels are safe until the Front Man turns off the lights, removing his advantage. With time running out, Sang-woo pushes Player 017 to his death, revealing the last trick panel. Only Gi-hun, Sang-woo, and Sae-byeok complete the game; at its conclusion, explosions shatter the remaining panels and injure the three players.
As finalists, Gi-hun, Sang-woo, and Sae-byeok are given a change of outfit to formal clothes. Sae-byeok hides a severe stab wound she received from the glass bridge explosion. After eating dinner, each player is left with a steak knife. Gi-hun suggests to Sae-byeok that they should ally against Sang-woo, realizing he will go to any length to win. Sae-byeok instead begs Gi-hun to promise that whoever wins the game will look after the other's loved ones. Gi-hun goes to kill Sang-woo when he falls asleep, but Sae-byeok stops him, telling him he is not a murderer. Sae-byeok's injury worsens, prompting Gi-hun to call for help. When Gi-hun is away, Sang-woo kills Sae-byeok and the guards arrive only to collect her corpse. Furious and heartbroken, Gi-hun tries to attack Sang-woo but is prevented by the guards. Meanwhile, Jun-ho makes it to another island but is quickly tracked down by the Front Man and the guards. To Jun-ho's shock, the Front Man reveals himself to be his brother, In-ho, who tries to recruit him. When he refuses, In-ho shoots Jun-ho in the shoulder, causing him to fall back over a cliff edge into the sea.
One Lucky Day
In the final eponymous Squid game, Gi-hun defeats Sang-woo after a brutal fight but refuses to kill him; he begs Sang-Woo to stop the game using the third clause. Sang-woo instead stabs himself in the neck, and asks Gi-hun to take care of his mother before dying. Gi-hun is returned to Seoul with a bank card to access the prize money, but discovers his own mother has died. A year later, Gi-hun remains traumatized and has not touched his prize money. He receives an invitation card from his gganbu, and finds Oh Il-nam on his deathbed. Il-nam reveals he created the game to entertain bored ultra-rich people like himself. He chose games he played as a child, and participated in Gi-hun's group out of nostalgia. As they talk, Il-nam wagers with Gi-hun on whether an unconscious man lying on a street corner will be helped before midnight. The man is saved, and Il-nam dies shortly after. Gi-hun retrieves Sae-byeok's brother and has him looked after by Sang-woo's mother, also giving them a share of the prize money. As Gi-hun travels to the airport to reconnect with his daughter in Los Angeles, he sees the same game recruiter playing ddakji with another desperate player, but manages only to get that player's invitation card. He calls the card's number before boarding his plane, demanding to know who is running the games. He is curtly ordered to get on the plane, but he ends the call and returns to the airport terminal.