As the seasons pass, the Lion Guard grow into teenagers and Kion's Roar of the Elders increases in power. Kion unleashes his Roar again when Simba and Rafiki approach them. Rafiki states that the Roar is getting stronger as Kion gets older. Simba agrees and announces that it's time to defeat Scar once and for all. With Jasiri offering to help as well, they come up with a plan and gather the Pride Landers to strike the Outlands that night. However, Shupavu and Njano overhear their plan and slither away.
Meanwhile, at the destroyed Kilio Valley, Janja sits on a rock, talking to Cheezi and Chungu about how Kion's Roar is now more powerful than ever. Cheezi and Chungu are tired of fighting and losing to the Lion Guard and Janja begins to wonder if they might be on the wrong side, wondering if Jasiri was right all along and that there could be a new way to go. When his friends ask what he's speaking about, Janja shrugs their concerns off. However, Nyeusi and Nyata overheard him questioning his loyalty to Scar.
In the Outlands Volcano, Ushari is speaking with Scar, having the skinks' received news of Janja's possible reformation and the impending attack from the Pride Lands. Scar is undeterred, and it isn't long before Shupavu comes to Scar with Janja in tow, as Scar had requested. When Janja walks in Scar orders him to go the Lair of the Lion Guard and not let the Guard out. He explains that he wants the hyenas to distract them, and voices his concerns to Janja that Kion could destroy him if he uses the most powerful Roar he can muster in the caldera, hence why he needs to strike back. Janja obeys and leaves, with Shupavu following him after being ordered to fetch Reirei, Kiburi, and Mzingo. Scar assures Ushari that he's always two steps ahead.
Later that night, Zazu alerts the royal family that Hadithi with their ally, Shujaa, has come to help. With all the Pride Landers gathered together, Simba asks Kion to lead the battle in the morning. Kion confesses that he has concerns over how to defeat Scar, but Simba remains positive. Soon, Kion and the Guard start to think to talk of the building anticipation that this is the last night before the big battle, with Kion needing to speak with someone beforehand.
Nearby, Shupavu leads Janja's Clan into Pride Lands. They spot Goigoi who is carrying dry grass and question him. They discover that it's all part of Scar's master plan, and Shupavu continues leading them to The Lair of the Lion Guard. Meanwhile, Kion speaks to his grandfather Mufasa. He is worried that there's no way to defeat Scar. Mufasa is as confident as Simba that Kion can handle it, and reminds him to be himself, and not to fight fire with fire.
Back near Pride Rock, Janja's clan enters the Lair of the Lion Guard so they can trap the Guard, grabbing Ono's attention. When the Guard enters, Kiburi's Float adds more dry grass and sticks to the entrance, and before long, Mzingo's Parliament sets them alight. It doesn't take long before the lair fills up with smoke. Janja escapes the fire and, realizing Scar has betrayed him, goes to find Jasiri, the only one who will trust him.
Ono flies out of the lair to discover the entirety of Pride Rock is on fire, and Kion orders him to save his family. With help from Anga and Hadithi, the royal family is lifted to safety one by one.
With the time of the essence, Beshte remembers that the lake in the pool leads to Lake Shangaza just outside the lair. When Janja's clan asks to follow, Kion allows them to do so, telling them that he respects all creatures, even those who don't follow the Circle of Life. With Fuli ridding on Beshte's back, the Guard and Janja's clan swim through and out the other end, where they notice the severity of the fire.
With the royal family safe outside Pride Rock, Scar appears on top of the burning Pride Rock and announces that the Lion Guard wasn't so lucky to escape. But Kion and the Guard show up with Janja's hyenas by their side, but Scar says that there's no way to defeat him before vanishing. The herds begin to panic when Jasiri and Janja arrive. Janja offers advice on how to defeat Scar based on what he heard earlier from him, which Kion takes. They set up the plan and get some rest.
Back in the Outlands Volcano, Scar tells Ushari of how he lied to Janja and even though Kion's roar can defeat him, it won't work the way Scar told Janja. Scar's actual plan is to get Kion to use his most powerful roar in the caldera which would cause a massive volcanic eruption that would turn the Pride Lands into a wasteland and kill all of his enemies in one fell swoop. He then proceeds to tell Ushari the story of how he got his scar when he led his Lion Guard.
A strange lion fooled him, leading him to a snake that attacked and gave him his scar. The strange lion promised to cure the venom provided he did everything he ordered. Enraged, Scar used his Roar and destroyed them instead. When he returned to Mufasa, the King merely nicknamed him "Scar", causing him to resent his brother. This was what triggered Scar to destroy his Lion Guard, lose the Roar, kill his brother and take over Pride Rock before Simba defeated him. He tells Ushari that the next part of the plan will involve him, and explains his plan.
The next morning, Kion leads the assault on Scar's army. They fight against Reirei's Pack, Kiburi's Float and Mzingo's Parliament as Kion and the Guard aim for the volcano. Scar orders Shupavu's Group to slow them down, all to give the impression that they don't want the Guard to reach him.
The Lion Guard soon enters the volcano, and make their way to Scar. Scar orders Ushari to charge at Kion, resulting in a scar of the cub's own. He then proceeds to taunt Kion with Jasiri's signature catchphrase: "Sisi ni sawa", to convince Kion how alike they are, but Mufasa's voice rings in Kion's mind, reminding him not to fight fire with fire. Kion refutes Scar's remark, and, finally realizing what his grandfather meant, offers Scar his forgiveness, much to Scar's shock.
Kion tells Scar that he cannot judge him for his previous actions in everything and that only the Lions of the Past can do it. He takes a deep breath and blows softly to the air, prompting the Great Lions to appear in the sky. They send down rain from the heavens that vanquish Scar away, but not before he warns Kion that the Roar is a curse. Desperate, Ushari tries to avenge Scar by attacking Kion again, but Bunga tackles him into the lava. Ono saves Bunga from falling to his death, but Ushari meets his fiery fate.
Kion and the Lion Guard emerge from their mission and the Pride Landers and the redeemed Outlanders cheer. The Outlanders are also shocked that they no longer have a leader, but Janja takes the initiative to say that Jasiri should be in charge of the Outlands. Accepting, Jasiri announces that she is certain they can all get along if they all respect the Circle of Life, and Janja announces that he will be helping her keep everyone in check, by joining her clan. Knowing that Jasiri will take care of Janja, Reirei gladly accepts. When Mzingo declares that a vote would be more suitable, but Cheezi and Chungu growl at him, causing him to accept the offer. On the other hand, Kiburi is indifferent to the new order, adamant that crocodiles can fend for themselves.
Suddenly, Ono starts looking around him unable to see anything. Realizing Ono has become blind, the Lion Guard decides to bring him to Rafiki for help. With everything settled in the Outlands, Kion thanks Jasiri once again before leaving. On their way back, though, Kion comes upon a nearby pond and notices his new scar. Thinking on Scar's final words, Kion falters if he is destined to turn evil too as his scar begins to hurt. Bunga then returns to his friend, only for Kion to roar angrily and send him flying. After finding Bunga, Kion becomes distraught and remorseful, realizing he used the roar on his best friend. Now worried, Kion realizes he needs to see Rafiki along with Ono.
They return to the mandrill at the Lair, who says there's nothing he can do about Ono's eyes, which have become damaged due to the vog from the volcano: only time will tell if it will heal. When Ono starts getting discouraged, everybody praises him for saving Bunga's life, saying he embodies all the Lion Guard's best qualities. Realizing he is the Lion Guard, Ono briefly laughs before falling asleep, exhausted.
Rafiki then turns towards Kion. While he notes the scar itself cannot heal, he will need a cure for Ushari's venom. To this end, Rafiki offers Kion Tuliza, temporary relief for Ushari's mind-altering venom, though he notes it is not a cure. Kion's family then arrives, becoming worried about seeing the scar. Rafiki says that if the venom in Ushari's bite stays inside Kion for too long, he will no longer be able to tell the difference between right and wrong. This will make eventually turn Kion, evil like Scar.
While Rafiki reveals there is no cure for Kion in the Pride Lands, he does suggest that Kion goes to the Tree of Life. Makini explains that the Tree of Life is a sanctuary for ailing or endangered animals, having visited it in her past when she met Rafiki for the first time. Agreeing, Rafiki also reveals it to be the place where the Great Lions of the Past told him that Makini would become the next Royal Mjuzi. Rafiki then points to a nearby map, explaining that the road to the Tree of Life is marked by a set of landmarks, telling them to follow the painting in the rocks on their way there.
Having looked at the map, Ono then wonders if the Tree of Life could help him, too. The rest of the Lion Guard agree to travel with Kion, noting that with Scar gone and Jasiri in charge of the Outlands, there is finally peace in the Pride Lands and they won't be needed as much anymore. Simba and Nala agree as well, noting it's alright for the Guard to travel with Kion, given everything he has done for the Pride Lands.
However, when the subject of the Keenest of Sight comes into play, Ono suggests Anga as his replacement. Though Anga is hesitant to take the role, Ono encourages her to accept, noting she can see just as well as he did and fly even higher. Accepting her new role, Anga stands still as Kion declares her the new Keenest of Sight, placing the Mark of the Guard on her shoulder as Ono's Mark fades away. As Ono mourns his loss on the team, Kion reveals that, even if he is no longer the Keenest of Sight, he still wants Ono in the Guard. In honor of Ono's intelligence, Kion creates the role of the Smartest for him, allowing him to stay in the Guard.
Now with six members, the Lion Guard gets ready to leave the Pride Lands to the Tree of Life. Ono reveals he has memorized Rafiki's map from the Lair, allowing him to keep track of their progress. At this point, Makini reveals she is joining their trip, keeping a stock of Tuliza in the gourd on her bakora staff, which she will use to activate the moja kwa moja stones they will find on their way. Kion says his goodbyes to his family, advising Kiara to keep Tiifu and Zuri in check while they're gone, while Simba and Nala wish him good luck, hoping they will be back in time for Kiara's first hunt.
As the Lion Guard makes its way out of the Pride Lands, they are met with a surprise: all the animals they have helped and met throughout the seasons have gathered to wish them goodbye and good luck on their journey, bowing and saluting with respect as the Guard passes them by. On their way out, Kion briefly stops to talk with Thurston, who pompously says the zebras will protect the Pride Lands while they're gone. Playing along, Kion thanks him for his help.
On a cliff overlooking the Pride Lands, Kion stops to take one last look at his homeland and the friends and family he is leaving behind. Giving a reassuring smile, Kion repeats the catchphrase of the Lion Guard "Till the Pride Lands end..." Knowing the Pride Lands will survive without them, Kion makes his peace and begins moving forward, facing the future as he starts the journey to the Tree of Life for the healing he and Ono need.