Makini and the Lion Guard are in The Lair of the Lion Guard, discussing Scar's defeat at the paws of Simba ahead of the Kumbuka celebration, with Bunga arguing that his uncles Timon and Pumbaa were just as responsible for saving the Pride Lands as Simba. Makini becomes visibly annoyed at Bunga for detracting from Simba's efforts, and the Lion Guard universally agree that if Scar was still around, things wouldn't be very good.
Meanwhile, in the Outlands Volcano, Scar has gathered his army. Janja, Reirei, Kiburi, Ushari, and Mzingo listen intently as their leader announces that they will be taking down both Simba and the Lion Guard. When questioned how, Scar introduces them to a friend who will be assisting them - Sumu, a scorpion. The sight of the scorpion startles Janja, who leaps onto Reirei and soon Kiburi in fright. When Sumu's orders have been confirmed, he departs. Scar declares that the day will be known as the day he returns, and defeats both Simba and the Lion Guard.
At Pride Rock, many Pride Landers have gathered for the Kumbuka celebration. As he starts the celebration, Sumu sneaks to the top of Pride Rock and stings Simba on his tail. Although Simba feels the sting, his body reacts almost immediately to the poison, and he collapses while Sumu sneaks away unseen. When everyone rushes to his aid, Simba weakly directs them to his tail, and Rafiki inspects the bloated sting mark. When he reveals it to be that of a scorpion, Kion sends Ono to search for one. Ono locates Sumu on his way back to the Outlands, and the Lion Guard chase after him. But before they can catch him, Sumu enters a hole which leads to the Outlands. Having made sure that the danger has left the Pride Lands, the Guard return to Pride Rock.
They find Timon and Pumbaa sobbing uncontrollably, unaware of exactly how bad Simba's condition is. When they enter Pride Rock, they find Nala and Kiara comforting the barely conscious King, while the Royal Mjuzi and his apprentice use a variety of mixtures that they hope will cure Simba. Suddenly, Rafiki remembers the cure - volcanic ash. However, he warns them that it must be recovered by sundown. With the closest volcano being the one in the Outlands, the Lion Guard offer to retrieve it. Rafiki hands Bunga a gourd to place the ash in, but Bunga's cockiness results in the gourd breaking. Rafiki decides to send Makini out with the Guard, and she attaches another gourd to her Bakora Staff. With everyone set, they head out.
In the Outlands, Scar hears from Sumu that the plan was a success, and knows that the Guard would be on their way. He orders Ushari to get the others ready. Proceeding with the plan, Kiburi's Float waits for the Lion Guard. When they arrive, Kiburi refuses to move without a fight. With time of the essence, Kion sends Beshte to deal with the crocodiles. The hippopotamuses tackle Tamka, and the force of the tackle slams him into the rest of his float. The Guard hurries through. The Guard get a little further but bump into Reirei's Pack. Kion turns to Bunga, who leaps onto some nearby rocks and bounds over the jackals, farting on them. The stench keeps the pack from stopping them and the Lion Guard runs past them while Fuli comments that she's glad that Bunga is on the Guard's side.
Worn out, 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 with Kion knowing that Scar's plan has failed, 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.
Trapped, Kion resorts to the final option. Scar warns him that using the Roar will cause the volcano to erupt, but Kion responds by telling Scar that he doesn't know as much about the Roar as he does, before using it in a similar manner to how he saved his mother from Makuu's float. He continues to use the Roar on Reirei's pack, and finally, Janja's clan, just before leaving. Scar demands that his followers get the gourd, and sends Mzingo's parliament out to retrieve or destroy it.
During their escape, Makini stumbles, giving the vultures time to attack. Ono, Fuli and Bunga race back to help her, with Ono diving in and scattering the vultures before they can peck the gourd away from her staff. Mzingo sends two of his vultures to deal with Ono, while he and Mwoga aim for the gourd. They manage to knock the staff away from Makini, and the staff falls bottom-first into the lava. Fuli and Bunga tackle the final two vultures, and Makini grabs the gourd. Seeing her having difficulty removing the gourd, Ono swoops in and severed the vines holding it. When the vultures return, Kion uses his Roar once again to send them away, and they hurry back home.
Back in the volcano, Scar glares over at the defeated army, stating that the war has just begun. As the Lion Guard race across the savannah, they start to worry about making it back in time. With Fuli being the only one fast enough to reach Pride Rock before sundown, Bunga offers to carry the gourd on her back. The offer is accepted, and Fuli speeds away. Nearby, a distressed Rafiki peers over the Pride Lands, fearing for his friend. He enters Pride Rock to check up on Simba when Fuli and Bunga arrive. When Fuli trips slightly, Bunga is propelled forward. Pumbaa manages to grab Bunga on his back, and Rafiki is able to catch the gourd before it breaks.
When Makini and the rest of the Lion Guard arrive, Kion is dismayed to find his father still on his side, with his mother and sister deeply upset. He gently nudges his father, who weakly acknowledges his son. Nala explains that, although weak, Simba will be fine. Kion thanks Rafiki, who reminds him that the Lion Guard deserve just as much of the credit. When Timon and Pumbaa start to cry in happiness, Rafiki ushers everyone out, stating that the King needs to rest. As they leave, Kion tells Rafiki that he needs to speak with his father. Rafiki suggests speaking later, so the cub tells him about how they saw Scar, and how he caused the sting in the first place. Worried, Rafiki agrees that, when his father is better, he will need to speak with him.
Kion joins Makini, Fuli, Beshte, Bunga, and Ono at the edge of Pride Rock, looking over the Pride Lands. He reminds them of their duty and states that, although it will be tough, they will defeat Scar.