After Kion uses the Roar of the Elders to protect his best friend, Bunga from Cheezi and Chungu, Janja gathers his clan together inside of the den and proceeds to scold Cheezi and Chungu for allowing themselves to be intimated by the roar of a lion cub.
After Rafiki informs Bunga of his immunity to snake bites, Bunga mistakenly interprets this to mean that he is immune to everything, and after irritating a porcupine, who tells him to play in a volcano, decides to do so.
There, he is confronted by Janja, Cheezi, and Chungu and two other members of his clan. But before they can attack, Fuli rescues him from their clutches, and the rest of the Lion Guard soon catch up to assist them. In the end, Janja, Cheezi, and Chungu are left stranded on a log in the middle of the river of lava.
Cheezi and Chungu are seen standing in the Outlands with the volcano seen in the distance. Janja then finds Mwevi and Mwezi in his den and chases after them. Later on, after the hyena trio is defeated by the Lion Guard, they head off in the direction of the volcano.
Janja and Ushari discuss a plot about how to bring back Scar and defeat the Lion Guard. While Ushari tries to use Makini's staff to summon Scar, Janja and his hyenas kidnap Kiara in an effort to get Kion to roar over the volcano. When kidnapping Kiara didn't work, Janja taunted his archenemy by telling Kion they'll threaten everyone he cares about. Having his anger get the better of him, Kion uses the Roar of the Elders which causes the volcano to erupt and Ushari tries to talk to Scar but to no avail. The cobra informs Janja that their plan seems to have failed, Janja in anger throws Makini's staff into the pit and a spirit rises with Janja trembling at who he thinks it is and the spirit confirms himself to be Scar.
Scar hears from Ushari's friends that the guard has accidentally woken up Makuu and his float but wants to be informed of any changes. Then the skinks inform Scar that Kiburi has called for a Mashindiao and the spirit remembers that the event causes a crowd of large animals including the Royal Family. He tells Ushari to hatch a plan in Kiburi's head during the Mashindiao event but when it fails Kiburi and his followers are banished to the Outlands. The crocodile plans to eat Ushari but the cobra tells him there is something he wishes to see when Kiburi comes to the Volcano he hears of Scar's plan to take over the Pride Lands once again and the Circle of Life for good.
After getting defeated by Jasiri, Janja, Cheezi, and Chungu return to the volcano and confess to Scar about what happened. He states that she is The Worst Hyena We Know, which in turn causes Scar to order them to get rid of her.
Scar speaks with Janja and Reirei in the volcano, telling them explains that their first mission was merely a test, to see the Lion Guard would force to split up. And telling them all about his latest his plan eliminating Rafiki.
Scar speaks to all of the leaders about his next plan to finish Simba once and for all. Soon a Scorpion named Sumu comes and goes to Pride Rock to sting Simba. Later the Lion Guard reach the volcano. Before they can ascend to get the ash, Janja's clan arrives. The hyenas attack, and the Lion Guard defend themselves, with even Makini helping out (albeit accidentally). They make their way up the volcano, where Kion observes the slowly setting sun. When they finally reach the highest point, Ono guides them to the volcanic ash, and Makini swiftly pours a handful into her gourd, sealing it shut and re-attaching it to her staff. Just as they are about to leave, Scar rises from the lava. Kion recognizes his evil great-uncle, but refuses to engage with him, reminding his friends that they need to get back to his father. Scar reveals that he was behind the attack, leading the Lion Guard to work out that the other attacks were planned as well. Scar commends them for their cleverness. The Lion Guard start to leave regardless, but the fallen King calls for his army. Suddenly, Ushari, Shupavu and her group of skinks, and Kiburi's float emerge. They approach the Lion Guard and, despite Bunga's eagerness to take them on, Kion leads them the other way, not wishing to waste any more time. But Janja's clan block their other pathway. When the final path is blocked by Reirei's pack, the Lion Guard realize they are surrounded. As Kenge joins the skinks and cobra, Makini and the Lion Guard start to back away to a ledge hanging over the lava. Thinking quickly, Kion orders Ono to find a way out, but the egret is knocked down by Mzingo and his parliament. Kion decides to use his Roar so they can get back to the Pridelands.
Kinyonga goes to the Outlands to spy on Scar he claims his next to Janja to enter the Pride Lands, reach the riverbank above Hakuna Matata Falls at night, and push the rocks into the river until it stops the water from flowing. When Janja questions what that will do, Ushari tells him that this will cut off a major water supply during the middle of the dry season.
Scar hears that a gorilla named Shujaa is attacking the Pride Lands while trying to help the Lion Guard. He soon orders his army to keep on attacking so that way the gorilla will do the wrecking for them.
Scar has been seen in the volcano explaining how he got his scar in When I Led the Guard. Later the Lion Guard goes to defeat Scar. 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 melts 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.