In the Undercity of Zaun, the boy genius Ekko witnesses an explosion that destroys a giant spire owned by the Poingdestre chembaron family. Amid the ensuing chaos, he searches for his friend Lem, whom he was supposed to meet near the tower. Finding Lem trapped under debris, he rescues him. Lem tells him that a bully with a giant hammer stole his Timewinder, but Ekko reassures him that he'll retrieve it and sends Lem back to the Hub, their hideout near the market. Ekko tracks the Timewinder to Scary Janet, who is seen empowering her armor with strange orange crystals she obtained from the ruins of the tower. After defeating Janet and reclaiming the Timewinder, he returns to the Hub to check up on the rest of his friends.

He meets up with Red and Aximander, who mention that the Poingdestre twins have been hiring kids to scavenge the remains of the spire and that Clan Ferros guards from Piltover have been spotted in Zaun. Ekko decides to check on his parents before investigating the spire. Along the way, he learns that his friend Elie was evicted by the Poingdestres. Fortunately, she has found a new apartment closer to the market with the help of Corin Reveck, a craftsman who relocated to Zaun after spending a fortune to save his daughter, Orianna. Elie tells him about her plans to open her clothing store and asks Ekko for help in finding inspiration through kaleidosalts and kaleidolilis.

When Ekko visits Corin, he witnesses him being harassed by the Inx, a street gang inspired by Jinx. Ekko scares them off, but Corin mentions that they have stolen multiple figurines and offers some cogs to Ekko if he can retrieve them. He also meets up with his friend Rungs, who returns some equipment he had borrowed.

When he arrives at his parents' apartment, he finds a hooded figure waiting for him in their garden. The stranger says he needs Ekko's help, but Ekko is wary of him for spying on his parents. When the stranger refuses to leave, the two engage in a fight, with Ekko emerging as the victor. The hooded figure then reveals himself to be Ekko from the future and explains that he needs his past self's assistance. He mentions that he traveled through time to investigate the spire collapse, but before he can reveal more, Ekko's parents, Wyeth and Inna, return.

Future Ekko urges him to rendezvous at the Wattlick Power Plant in Factorywood, where he'll explain everything. Afterward, Ekko checks in with his parents, who inform him that they were far from where the spire fell. They note that orange muck was splattered all over Factorywood, the site of the collapse.

As Ekko makes his way to the power plant, he spots Vale Poingdestre ordering her lackeys to search for more Syntixi Crystals near Factorywood. He also encounters a Golden Scrambot, which he races to catch. After capturing it, he smashes it and decides to take its remains to Aximander when he is finished.

When he meets up with his future self, Future Ekko explains the origins of the Syntixi crystals. In Piltover and Zaun, the Hextech Crystal trade is dominated by Clan Ferros, but due to their rarity, they began experimenting with creating synthetic crystals. They struck a deal with the Poingdestre family to manufacture the crystals in Zaun, but the Poingdestres decided to create their own lab-ripped compound called Syntixi. Syntixi is just as powerful as Hextech but much more volatile. When word reached Clan Ferros of their betrayal, they attempted to shut down their operation, but the Poingdestres had already amassed an arsenal of Syntixi. When the Poingdestre Spire exploded, their stash of Syntixi scattered across Zaun.

Future Ekko reveals that the Poingdestres and Syntixi cause great strife in the future, so he came back to stop them. Ekko agrees to help his future self, and they split up to search for the Syntixi. After collecting several crystals, Ekko comes face to face with Zarkon Poingdestre, the eldest child of the Poingdestre family. After defeating Zarkon, Future Ekko rejoins him. Ekko observes that Zarkon had gathered most of the Syntixi in the power plant, but Future Ekko has a plan to retrieve it. Before they proceed, Ekko wants to check on his friends and repair his gear.

Upon returning to their hideout, he sees Red leaving to hang out with some new friends, members of the Inx. Lem introduces him to Trevor, a dog he brought back and outfitted with a mechanical leg. Aximander explains that the scrambots are robotic vaults once owned by an eccentric inventor and, after his death, now roam the streets of Zaun. He asks Ekko to retrieve their contents as they contain artifacts related to the history of Zaun.

After Ekko catches up with his friends, Future Ekko informs him that he traced more of the Syntixi to the Sump, the lowest part of Zaun and the convergence of all its pipes. As he searches the Sump for crystals, Ekko begins to suspect that something is tracking him. He eventually encounters Drake Poingdestre, who mistakes him for one of their hired goons. Before Ekko can obtain the Syntixi, Drake realizes who he is and attacks. Ekko fends him off and forces Drake to retreat, allowing him to secure some Syntixi.

While searching for more Syntixi, a pair of Poingdestre goons taunt him. Suddenly, they are silenced, and Ekko only hears the sounds of claws on steel. When he goes to check on them, he finds them gone.

After collecting the last of the Syntixi, Ekko goes to rendezvous with his future self. While waiting for him, Ekko senses something closing in on him, and suddenly he is attacked by The Wrath of Zaun. He races through the Sump, attempting to escape the beast, but to no avail. Cornered, Ekko engages in a fierce battle with Warwick in the Sump Sewers. Ekko manages to incapacitate him just in time for Future Ekko's arrival.

His future self urges him to finish off Warwick, citing the numerous people he will kill in the future, but Ekko hesitates. Before they can decide what to do with Warwick, more Poingdestre goons arrive, forcing the Ekkos to retreat while Warwick regains consciousness.

Once they reach a safe distance, Ekko confronts his future self, who wants to show his past self something important. Future Ekko takes him back to their parents' apartment and gives him his chronoport, allowing him to travel to the future. In the future, Ekko witnesses that Zaun has been overtaken by the Zaun Gray, a chemical fog that blankets the city. A local watchman advises him to get inside due to the city-wide curfew, prompting Ekko to inquire about how things deteriorated so much.

The watchman explains that Piltover increased their dumping into Zaun after Clan Ferros discovered the Poingdestres manufacturing more Syntixi. Now, the cities are on the brink of a Hextech and Syntixi-fueled civil war. Ekko decides to check on his future parents and finds them packing their belongings. His father explains they plan on leaving Zaun, as the city has become too polluted, making survival impossible despite their hard work. However, they had left Ekko's belongings at the apartment in case he ever returned. Ekko is bewildered as to why his parents are leaving without informing him, but they reveal that they didn't part on good terms during their last conversation and ask him to let them depart in peace.

Ekko attempts several times to rewind their conversation and convince his parents to stay, but ultimately decides to let them go.

When Ekko returns to the present, he comprehends why his future self came back, but he doesn't believe it's right to try and control everything through time. He tells his future self that he needs some space to think, while Future Ekko heads out to investigate a shipment of Syntixi. Ekko then returns to the Hideout to speak with his friend Aximander, who recommends getting a breath of fresh air from 9 Corina Veraza's cultivair, a privately-owned greenhouse used to farm oxygen, located in Entresol West.

Upon arriving at the cultivair, he witnesses Zarkon demanding to speak with Corina, but she dismisses him. Realizing she is involved in the Syntixi plot, he decides to investigate.

Once inside, Ekko informs his future self about Corina's involvement with both the Poingdestres and Clan Ferros, having seen some of their guards. However, he also spots his friend Red inside and wants to ensure her safety. He overhears Corina being interrogated by a Ferros Captain and learns that she struck a deal with the Poingdestres to conduct experiments on Syntixi but returned it when she discovered how volatile it is. The Captain doesn't believe her and suggests she tells the truth before their intelligencer arrives.

After defeating the Captain, he shares his findings with Future Ekko and decides to locate Red before meeting up with him. He finds Red at the top of the cultivair with the Inx, where she introduces him to Chadd, her new boyfriend. Although Ekko isn't particularly fond of the Inx, he lets Red hang out with them and proceeds to rendezvous with his future self on the roof.

When he reaches the roof, he is intercepted by Camille Ferros, Principal Intelligencer of Clan Ferros. As the largest proprietor of hextech technology in Runeterra, they track the movements of all the hextech that exits their labs, including the splinter in his Zero Drive. While they previously considered him nothing but a nuisance, his recent interference in their investigation of the Poingdestre family has made him a threat. So, they sent Camille to put an end to him.

Ekko manages to hold her off until his future self arrives and informs her that they have been investigating the Syntixi crystals as well. Realizing they share a common enemy in the Poingdestres, Camille proposes they work together. While the Ekkos intercept a shipment of Syntixi aboard the Chaincrawler, the Poingdestres' train, Camille will ensure none of the siblings interfere.

Ekko doesn't like that his future self aligned them with Clan Ferros, but he reassures his past self that this is only a temporary alliance.

Once Future Ekko locates the Chaincrawler, he instructs his past self to commandeer it while he looks for a way to safely dispose of the Syntixi. Aboard the train, Ekko battles Vale and Drake Poingdestre to wrest control of the Syntixi. Future Ekko eventually joins him in the control car, where he outlines his plan to get rid of the Syntixi. His plan involves detonating it at the halfway point between Piltover and Zaun to destroy both the Syntixi and Piltover's horde of Hextech, thereby preventing either side from powering an army and averting a civil war.

Present Ekko is deeply concerned about the potential casualties of an explosion of that magnitude, but Future Ekko believes that the ends will justify the means. Ekko is furious that his future self would consider such a drastic action and refuses to let him take the Syntixi. However, Future Ekko knocks him out and departs with the crystals.

Upon returning to the market, Corin informs him that he has seen the Inx moving suspicious boxes into the Fenlox Theatre and suspects they are planning something. Recognizing the mounting challenges he faces, Red invites Ekko to unwind at the Inx show at the Fenlox. Initially hesitant, Ekko eventually agrees. Red tells him that Chadd and the Inx have something special planned for this concert and wants to meet up with him first.

As Ekko wanders around the theatre, the doors suddenly lock, and the theatre begins to separate and float into the sky, carried by giant balloons and motors.

Ekko overhears the Inx noting that this was all done to get Jinx's attention. He eventually spots the Loose Cannon herself, now sporting Syntixi weapons, and firing them at the Inx. Despite their questionable choice in role models, Ekko can't let Jinx hurt them. He tries to warn the Inx about the danger they are in, but they refuse to let him scare away their idol.

As he makes his way towards Jinx, he sees Red and Chadd arguing. Red is angry that Chadd didn't tell him about the theatre or the explosives, leading Chadd to break up with Red.

Ekko eventually finds Jinx, who only came to destroy the Inx for copying her. They engage in a battle, and Jinx reveals that the Poingdestre twins offered her Syntixi weapons in exchange for causing a distraction so they can loot a mansion. However, she declined and stole the weapons anyway. During their fight, Jinx gains the upper hand and unleashes a barrage of rockets. Fortunately, Camille arrives and cuts down the rockets with her blade legs. After the two of them defeat Jinx, Camille asks Ekko if he has any way of reaching his future self. Ekko feigns being attacked by Camille, forcing his future self to save him. Ekko then informs Camille about his future self's plan to create a Syntixi bomb, but he doesn't have enough Syntixi yet.

Once his future self departs, Ekko invites Camille to the hideout as they await Future Ekko's next move. While recovering at the hideout, he talks with Red about what happened at the theatre, and they reconcile. As Ekko shows Camille around their base, she discovers a secret room, which is Future Ekko's hideout. Inside, Camille deciphers his journal and learns that this is his final chronoport and his only chance to stop the impending war.

Ekko spots markings around the August Fairgrounds along with the Poingdestre crest and believes that's where the last stash of Syntixi resides. He sets out to stop him, while Camille stays behind to gather more information about his plans. As he makes his way to the fairgrounds, he spots his parents running errands. Although he can't make it home for dinner tonight, he promises to always come home.

As Ekko searches for his future self in the fairground, he hears the distinct howl of Warwick. He attempts to reason with his future self, but Future Ekko remains determined. When Ekko inquires about how he plans on transporting the Syntixi explosive to Piltover, Future Ekko reveals that he found the perfect location—the Clocktower, the halfway point between Piltover and Zaun. Realizing that his future self has already obtained the Syntixi, Ekko rushes out of the fairgrounds toward the clocktower. As he reaches the exit, he is intercepted by Zarkon. As they engage in a battle, Warwick arrives and attacks Zarkon. After defeating the chembaron, Warwick turns his attention toward Ekko. Ekko flees from the Wrath of Zaun, but Camille arrives and helps him escape, and together, they head towards Poingdestre Clocktower.

At the top of the clocktower, they discover that Future Ekko has enclosed the area with a time barrier, preventing Camille from entering. Ekko enters to confront his future self, while Camille waits for the barrier to deactivate. As the two Ekkos engage in battle, their temporal energy causes a tear in time. Present Ekko manages to gain the upper hand and finally persuades his future self that there's another way to prevent the war. Future Ekko then instructs him to rewind his Zero Drive, and together they widen the rift.

In the end, Future Ekko decides to dispose of the Syntixi by throwing the bomb into the rift and sealing it from the other side, even if it costs him his life. With Future Ekko gone, Camille eventually reaches the top and informs Ekko that her agents have captured the rest of the Poingdestre family and shut down their Syntixi manufacturing. She assures Ekko that he is no longer doomed to that fate, but Ekko remains uncertain about his future.

As Future Ekko hurtles through the rift, he activates his chronoport and launches the Syntixi bomb. With the bomb gone, he peers into the multiverse and views different timelines, including  Arcane,  Star Guardian, and  PROJECT. However, the most significant one to him is the one where his parents decide to stay in Zaun. With the portal closing, he steps through.

Future Ekko activates his chronocomm and leaves one last message for his past self. He apologizes for all that he has done and tells his past self that he doesn't have to become him. He encourages his past self to give his heart to the world to make it better and hopes that he can find forgiveness.

In the end, Ekko brings Red to his parents' apartment and shares with her the story of Future Ekko.