That Time I Got Reincarnated as a Slime season 4

As Shuna searches for Rimuru, the latter observes Tempest and sees how monsters and humans have been getting along. He has also made plans to build a train station for the new train and a bar with female elves on the Dungeon’s 95th floor. However, Rimuru also learns that the Dungeon has been having issues. Meanwhile, Hinata teaches class in Tempest, with Class S being among them, but when she catches Rimuru in the classroom, she throws him out. After Shuna finds Rimuru, she gives him a new type of dish: pudding that resembles Rimuru himself, which he shares with Veldora, Ramiris, Gard, and Masayuki. While talking about how to improve the Dungeon, Masayuki gives Rimuru some helpful ideas such as save points, bathrooms, restaurants, and tutorials, while Gard creates advertisements to attract adventurers with the promise of reward money. After Shion makes tea for Rimuru, Diablo accepts no reward for his help in taking control of Falmuth, but he does wish to have his own subordinates. Rimuru also has Souei keep an eye on Muese to find out more about his background. In Engrassia, Yuuki is unable to progress with his plans now that Rimuru is onto him, so he decides to be neutral towards Rimuru despite concerns from the Alliance, but he also continues his plans in the West. Meanwhile, Mariabel and Granville begin their plans to destroy Tempest in favor of a former Chosen Hero.

Team Lightspeed try out the Dungeon again. After clearing the 30th floor, Masayuki's fame rises. Rimuru, Masayuki, Ramiris, Grad and Veldora are pleased to see how the Dungeon is functioning. They also learn that another group called Team Green Fury is clearing the Dungeon quickly. Ramiris and Veldora are paid for their hard work. Also, merchants from the Western Nations are visiting Tempest. Private research labs for Rimuru and Ramiris are built in the Dungeon. Shion and Shuna visit Rimuru during this time to let him know that Grad needs to talk to him. Grad reveals that Team Green Fury have also cleared the 30th floor. It turns out that the party has been increasing their members and using save points before a boss fight, which will be a problem for the Dungeon. Rimuru also learns that the bosses on the 60th and 70th floors have been experiencing issues and the Crusaders training in the Dungeon are having their own problems, leading Hinata to take on the Dungeon herself. She managed to clear the entire Dungeon, but couldn't beat Veldora. Rimuru isn't willing to reward her though. He, Shion, and Shuna visit Adalmann, who lost to the Crusaders since he can't use Holy Magic anymore. After helping him develop new magic attacks, they provide him with his throne room. They still have to resolve the problem with Ramiris's Elemental Colossus, which can't respawn. Milim returns with Ranga and Gobta, having finished their training. She is shocked to learn that the dragons that she caught for the Dungeon have been beaten. Ranga and Gobta are rewarded for their hard work. Upon learning of Team Green Fury's progress, Rimuru makes new devices that create avatar monsters based on their thoughts and transfer their consciousnesses into those monsters, which they intend to use to stop Team Green Fury. However, they need to learn how to use their new forms first.

In the Kingdom of Seltrozzo, Maribel recalls her past as a corrupt noble from Earth, having died from old age. After being reborn in the Central World, she was given a special unique skill called Greed, which allows her to manipulate greed in others, though she doesn't use this power on Granville though. The two plan to bring peace to the world by conquering it. Back in Tempest, following issues with handling their avatars, Rimuru and his friends go to see Kurobe for new gear. They start training on how to properly handle their avatars in the Dungeon. Grad soon informs Rimuru about Team Green Fury's progress, and Raphael advices Rimuru to interfere with Ramiris's Mazecraft ability so he can learn about the team's stats. Once ready, the team proceed to stop Team Green Fury. The plan is successful. Team Green Fury leave following their loss. Meanwhile, Yuuki arranges a meeting with Maribel. Since defeating Rimuru proves nearly impossible now to due to Veldora's presence, Maribel comes with the idea to use Greed on Rimuru in hopes of controlling him. She also intends to use Yuuki for her own means, and begins her plan to lure Rimuru to Seltrozzo so she can set a trap for him. Meanwhile, Muese is murdered for his failures, leaving Souei shocked.

Rimuru and his friends continue to take on adventurers in the Dungeon using their avatars, which is attracting more challengers. Rimuru then gets a visitor: Frey, which makes Milim nervous since Frey is aware that she is still neglecting her duties. After Frey leaves with Milim, Shuna is also aware of Rimuru was doing this whole time. Grad arrives with a letter from the Council of the West, leading Rimuru to suspect that Team Green Fury were scouts sent by the Council. Meanwhile, Maribel resumes her plans to destroying Tempest using the Council as pawns. The Council holds a meeting to decide whether they should ally with Tempest or not; Hinata is also attending the meeting. Eventually, the Council decide to let Tempest join them after all, but they also have concerns about the Eastern Empire and Veldora. Hinata leaves the meeting afterwards, but Elric, the first prince of Engrassia, and his knight Reiner, want to speak with her. Eric asks Hinata to serve as his bodyguard during the upcoming meeting with Rimuru to see if he is trustworthy or not, as he is still skeptical of the monster leader. Hinata refuses due to her allegiance with Tempest, warning him against antagonizing Rimuru. Elric did not take this rejection kindly. However, Hinata also suspects that the meeting was set up. Rimuru prepares to attend the meeting with the Council with Shuna, Benimaru, and Souei, though Shion is unhappy that she has to stay behind, until Rimuru explains that he's leaving her in charge of Tempest.

Rimuru and his subordinates arrive in Engrassia to join the Council of the West. They first take a tour of the place and start by buying new clothes, though Rimuru ends up getting kids' clothes by mistake. At a restaurant, they meet Hinata again. After casting a soundproof barrier, Hinata tell Rimuru more about the meeting and warns him that some of the nobles are opposing Rimuru or planning to use him for their own means, leading them to suspect that the Eastern Empire is taking action against the recent events. Rimuru asks Hinata to investigate Dwargon for a possible invasion route. Recalling Muese's death, they suspect that he has connections to the Western Nations. Meanwhile, Maribel plans to sabotage Rimuru during the meeting so to prevent his alliance with the Western Nation and hopefully destroy him with the help of Johann, one of the Five Elders. Yuuki visits Maribel and questions her motives. The next day, Rimuru visits the Council's headquarters where the Council members talk to him about the festival and the train that he was developing. As the meeting begins, the Council's chairman Lester names three rules that must be followed when joining the Council. Rimuru agrees with the conditions, but upon reading the results, Rimuru reacts with hostility.

It turns out the Council is attempting to take advantage of the new train, Tempest's weapons and fortune, and the Dungeon, which Rimuru takes as an act of slavery. Raphael senses that someone is manipulating the Council, so Rimuru removes the mental influence, but he wonders who was responsible for it; he does not suspect Yuuki. Elric arrives with Reiner, Team Green Fury and their leader Vent, and Gaiye, having been summoned here by Gaban, another one of the Five Elders, to deal with Rimuru against Lester's wishes. The rest of the Council weren't expecting this. Hinata, who was also in the meeting, is also displeased with this idea, but Elric reveals that he planned to take down and control Rimuru, demanding the Council to help him. Elric uses an Orb of Domination, the same thing that was used to control Milim (even though she wasn't really brainwashed); it cannot control Rimuru, but Elric doesn't know that. Elric attempts to force Rimuru to wear it, but the Council turns against him, leading to Gaban's plan being exposed by Rimuru. Elric's group refuse to give up, so Hinata takes on Reiner while Gaiye attempts to fight Rimuru, but Shuna decides to take him on instead. Hinata easily defeats Reiner and Shuna swiftly destroys Gaiye's holy sword before taking him down using her magic. Raphael senses Glenda attempting to kill Elric from a distance for his failures, which Rimuru stops at the last second. They suspect that someone is trying to frame Rimuru for murder as Reiner and Gaiye are arrested. Elric ends his conflict with Rimuru after seeing that he isn't evil, but also sees that Gaban is responsible for the whole thing. He doesn't have proof, but Aegil, the king of Engrassia and Elric's father, arrives and clears things up, forming an alliance with Rimuru while disciplining his son for his actions and having Gaban arrested. This worries Johann, who was also in the meeting. The Council decides to accept their allegiance with Rimuru after all, but with more peaceful matters. Meanwhile, Souei confronts Glenda following her failed assassination attempt.

Souei fights Glenda, who manages to kill him, only for him to reveal that she had killed a doppelganger. Glenda finds herself surrounded by more doppelgangers and the real Souei, resulting in her capture. Rimuru has another meal with Benimaru, Hinata, and Shuna following the meeting before Souei arrives and tells them about Glenda. Rimuru deducts that she is an Otherworlder. They interrogate Glenda regarding her actions. Rimuru does not want to kill Glenda, but Hinata and Shuna remind him that they can just revive her. Rimuru takes over the interrogation and with Raphael's help, sees that she was also under the same influence as the Council and that Maribel is the mastermind behind the whole thing. Hinata deducts that Maribel and her grandfather were the ones who manipulated the Seven Luminaries in the first place. Glenda reveals that she is under a curse that forces her to serve the Rozzos and she will be killed if she disobeys. Rimuru removes the curse, setting Glenda free. She decides to ally with Rimuru in return and reveals that Granville, thought to be killed, is still alive, and gives further information about Granville's scheme and Maribel's influence on Yuuki. Meanwhile, Maribel learns of her plan's failure. Though Johann considers allying with Rimuru, Maribel refuses as she still wants to kill the monster king.

Yuuki agrees with Maribel's plan and Johann learns of Glenda's capture, whom they believe has been killed by Rimuru. Yuuki also reveals that he intends to investigate some ruins called Amrita located in Distave, Clayman's old domain, which houses a powerful reactor. Maribel plans to defeat Rimuru there, even though Milim will be with him too. Yuuki has something that once belonged to Milim, which they plan to return to Milim so to separate her from Rimuru and defeat them at the same time. Yuuki also forewarns them about Veldora, but Maribel is not bothered by that. Meanwhile, Rimuru builds a new reception hall in Tempest and Glenda now serves Souei. He and his friends then visit a restaurant in Tempest. Glenda tells them more about the Five Elders, Maribel, Gaban, Johann, and their schemes. She also warns them about the Blood Shadow, a group that works for the Rozzo family. Rimuru sends Souei to investigate the Five Elders and the Rozzo family while he plans to investigate Amrita, with Gobta deciding to tag along. Rimuru is aware that Maribel may be setting a trap there, so he plans to counter it. The next day, Rimuru's group begin their trip to reach Armita along with Milim. They have made magical guns for their trip. Kagali also plans to go to Amrita alongside Rimuru. Granville wonders if Yuuki is trustworthy and Maribel plans to use her Greed skill to control Rimuru. Rimuru, Shion, Milim, and Gobta meet up with Kagali, who has a team of explorers join them. Rimuru summons a gateway that quickly takes them to Distave. They are greeted by a group of elves who live here.

The group are led through Clayman's old castle before reaching the entrance to Amrita. Inside is an ancient city. Kagali remembers that the original city was once part of an Elven paradise until was destroyed by the dragon princess as a result of them citizens angering them, forcing them to evacuate to find a new home; Kagali was among them. That was when the ancient people arrived in the cave and built Amrita. The ruins also have population with an artificial sun. Rimuru decides to make the city a part of Milim's territory as long as she allows the citizens to stay in it. They reach the door to the lower levels which only Clayman could open. Since Rimuru has consumed Clayman, he can use the latter's powers, though the explorers are desperate to learn about it. Though they manage to unlock the door, they decide to rest of the night. The next day, the group enter the lower levels where they find a mural showing the ancient civilization. Rimuru remembers that Milim's pet dragon was killed during that time. They stop for lunch during this time before navigating through the lower levels and reaching another door with the same magic lock. Meanwhile, Maribel and Yuuki unleash a powerful dragon to attack Rimuru. This causes Rimuru to sense an intruder in the ruins. Milim heads outside to confront the dragon, revealed to be the very same one that Milim sealed years ago; Rapheal informs Rimuru of this. Golems attack Rimuru's group. Rimuru opens the door and moves everyone through. Rimuru deducts that the dragon is the Chaos Dragon. Shion, Gobta, and Ranga manage to defeat all the golem. Maribel arrives to confront Rimuru, revealing that she deems him a threat. With her is Yuuki, Gaiye, and an unnamed knight, who are all under her control. Kagali is shocked by Yuuki's betrayal as Rimuru's team prepares to fight them.

Shion fights Yuuki while Ranga and Gobta fight the unnamed knight, who seems to have a grudge against Rimuru. Gaiye attacks Rimuru, but the former is quickly killed. Maribel creates a Holy Field around Rimuru and summons backup, but this fails as Rimuru had known what she was planning and had his other subordinates take down her backup. Seeing that her plan to control Rimuru is being compromised, Maribel traps Rimuru using her Greed skill, but this proves ineffective on him. Maribel still refuses to give up, but she is overpowered. Yuuki defeats Shion and turns his attention towards Rimuru. Maribel uses this distraction to escape. Yuuki has a skill called Anti-skill that renders him immune to Rimuru's skills and magic, but Rimuru manages to defeat him. Kagali and the researchers convince Rimuru to spare Yuuki, but the latter reveals that he is now free from Maribel's control. After the unnamed knight is defeated and freed from Maribel's influence, Rimuru learns that he was formerly one of the Three Battlesages and he wants to avenge Glenda, who was thought to have been killed. The knight then lets go of his grudge upon learning that Glenda is still alive. Rimuru lets Yuuki and Kagali go after Maribel while Shion, Gobta, and Ranga go help the Dark Elves, and Rimuru himself goes to help Milim. Maribel tries to find the reactor in the ruins, but discovers nothing. Yuuki and Kagali catch up to her and reveal that they tricked her. Yuuki also reveals that he was just pretending to be under Maribel's control all along, having also created Anti-skill. He then proceeds to stab Maribel and resume his plans for world domination. Maribel tries one final attempt to attack Yuuki, but fails as Yuuki steals her Greed skill before she dies. Meanwhile, Milim is still battling the Chaos Dragon.

A flashback reveals the origins of the four True Dragons. One of them had a child with a human woman, and the child was given an elemental dragon as a pet. That child is revealed to be Milim, who hasn't given her pet a name yet. The dragon was killed by a nation in an attempt to control Milim, leading her to destroy the nation before being calmed by Ramiris and Guy. The dragon was resurrected into the Chaos Dragon, which forced Milim to seal it away before becoming a Demon Lord. Returning to the present, Rimuru helps Milim fight the Chaos Dragon; he is aware of its backstory. Milim is unwilling to kill her friend. One of the dragon's attacks hits the castle, but Shion, Gobta, Ranga, and the researchers manage to escape in time. Rimuru comes up with an idea to stop the Chaos Dragon without destroying its soul and convinces Milim to trust him. At this point, Maribel's influence on the dragon vanishes as a result of Yuuki killing Maribel, allowing Rimuru to reach its core. Milim finishes off the dragon, destroying its body and triggering a sudden explosion, which Rimuru contains. Rimuru also reveals that saved the dragon's soul by transferring it into a soul-containing device, but can't get it out though. Milim decides to name the dragon Gaia, transforming the device containing the dragon's soul into an egg with the help of a Master Core. Milim is grateful of Rimuru. Yuuki is pleased that things are going well, but Kagali finds his methods a little scary. Yuuki had intended to use Maribel as a scapegoat as he had known that the Chaos Dragon would by a problem to his plans if Rimuru was defeated. He also figures that Laplace didn't kill the real Luminous Valentine and that Granville has connections to her. However, he is also planning to meet Granville since he was also under Maribel's control in the first place. Meanwhile, Rimuru decides to turn Amrita into a tourist attraction. Raphael warns Rimuru that Yuuki has ulterior motives. Granville learns of Maribel's death and is devastated.