Simba and Rafiki are looking out over the Pride Lands, while Janja's Clan is in Kilio Valley. Kion speaks with the Lion Guard knowing that as long as they're around the Pride Lands won't end.
Anga arrives and announces that the Outlanders are attacking, so the Guard all go there to fight. The Lion Guard and the Pride Landers are shown to be busy defending the Pride Lands from the Army of Scar, with the help of Jasiri as well. 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, some skinks overhear their plan, and slither away.
In the Outlands Volcano, Ushari is speaking with Scar, having 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's Group 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's group following him after being ordered to fetch Reirei's Pack, Kiburi's Float and Mzingo's Parliament. Scar assures Ushari that he's always two steps ahead.
Later that night, Zazu alerts Simba and Kion that Shujaa and Hadithi have 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. Battle for the pride lands.
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 enter, Reirei and Kiburi add more dry grass and sticks to the entrance, and before long, Mzingo's parliament set 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.
With time of the essence, Beshte remembers that the lake in the pool leads to Lake Shangaza just outside the lair. When Janja's clan ask 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 grabbing onto Beshte, 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 weren't so lucky to escape. But he and the Guard shows up with Janja's hyenas next to 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 that Kion's Roar can't defeat him. He then proceeds to tell Ushari the story of how he got his scar when he led his Lion Guard. A lion fooled him, leading him to a cobra 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.
Reirei's pack, Kiburi's float and Mzingo's parliament, slowing them down 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 enter 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 proceeds to use Jasiri's signature catchphrase, 'Sisi ni Sawa', to remind Kion how alike they are, but Kion refutes his remark by offering Scar his forgiveness. He tells Scar that he cannot judge him for his previous actions, and that only the Great Lions of the Past can do it, 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, but Bunga tackles him into the lava. Ono grabs Bunga from falling to his death, but Ushari meets his fiery fate.
Kion and the Lion Guard emerge out and the Pride Landers cheer. The Outlanders are angry that they no longer have a leader, but Janja takes the initiative to say that Jasiri should be in charge. Jasiri announces that she is certain they can all get along, 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. Kiburi is adamant that crocodiles can fend for themselves. Mzingo declares that a vote would be more suitable, but the hyenas chase him away. Nearby, Ono is unable to see what's going on, and the Lion Guard decide to return to Rafiki for help. With everything settled in the Outlands, Kion thanks Jasiri once again before leaving.
On the way back, Kion falters as his scar begins to hurt. Bunga returns to his friend, only to be Roared at. Kion is distraught, but his friends suggest that he speak with Rafiki for some help.
They return to the wise old mandrill, who says that there's nothing he can do about Ono's eye, which has become damaged due to the steam, and that only time will tell if it will heal. However, for Kion's scar, he suggests a visit to the Tree of Life, since all he can offer him is Tuliza, a temporary relief for his scar. Ono wonders if the Tree of Life could help him, too. The Lion Guard agree to travel with Kion, but when the subject of the keenest of sight comes into play, Ono suggests that Anga replaces him. She is hesitant, but Kion declares her the new keenest of sight after a bit of encouragement from Ono. But Kion still wishes to have Ono on the Guard, but as the smartest.
Now with six members on the Guard, the Guard get ready to leave the Pride Lands. Simba and Nala wish them good luck and hope he can return before Kiara's first hunt. Makini also joins the Guard, keeping a good stock of Tuliza in her Gourds on her Bakora Staff. The Lion Guard leave with Pride Landers wishing them goodbye. Kion stops to talk with Thurston who says that the zebras will protect the Pride Lands while they're gone.
Prince Kion takes one final look back at his homeland before giving a reassuring smile and leaving in search of the healing he needs.
- Max Charles as Kion
- Joshua Rush as Bunga
- Diamond White as Fuli
- Atticus Shaffer as Ono
- Dusan Brown as Beshte
- Rob Lowe as Simba
- Gabrielle Union as Nala
- Eden Riegel as Kiara
- Landry Bender as Makini
- Bryana Salaz as Anga
- David Oyelowo as Scar
- Maia Mitchell as Jasiri
- Blair Underwood as Makuu
- Ana Gasteyer as Reirei
- John O'Hurley as Hadithi
- Christian Slater as Ushari
- Common as Kiburi
- Gary Anthony Williams as Mufasa
- Khary Payton as Rafiki
- Andrew Kishino as Janja
- Kevin Schon as Chungu and Thurston
- Vargus Mason as Cheezi
- Greg Ellis as Mzingo
- Dee Bradley Baker as Baby Baboon
- Christopher Jackson as Shujaa
- Lynette DuPree as Ma Tembo
- Michael Dorn as Bupu
- Nolan North as Tamka
- Meghan Strange as Shupavu
- Phil LaMarr as Goigoi
- Jeff Bennett as Zazu
- We see the members of the Guard grow up some, with Kion's mane becoming a little more developed, as well as gaining a deeper voice.
- We learn how Scar got his scar and the name Scar and why he became evil in this special.
- Janja and his Clan reform and join Jasiri's Clan.
- Scar is defeated for good.
- Ushari dies in this special.
- The Army of Scar disbands.
- Jasiri assumes leadership of the Outlands.
- Ono loses his sight.
- Kion is given a scar like his great uncle's.
- Anga replaces Ono as the Keenest of Sight.
- Kion creates a new position on the Lion Guard: The Smartest.
- Ono rejoins the Lion Guard as The Smartest.
- The Lion Guard and Makini set out for the Tree of Life, explaining where they were during the events of the second film.