On the Day of the Departed, the Ninja hurriedly travel across the Sea of Sand, en-route to the Ninjago Museum of History. Upon their arrival, Wu reminds them of the holiday's significance: for remembering fallen ancestors and cherishing family.
Dr. Saunders arrives, greeting Wu and the Ninja before presenting the museum's new exhibit, the Hall of Villainy, which houses life-sized mannequins of the major villains fought by the Ninja over the years, which includes one of Aria.
"why is their one of Aria?" Jay asked
"well... she was a villain that you fought" Dr. Saunders said
Their was also a plaque describing the rare Yin-Yang Lunar Eclipse. Cole finds a painting of Master Yang, as well as the Yin Blade, which is on display. Cole calls out to Saunders and asks what the weapon is, but realizes nobody is paying attention to him.
Cole hears Yang's voice imploring him to "close the circle. Realizing he's fading away as one of the departed, Cole takes the Yin Blade and sets off to face Yang.
The Ninja divide for their own personal memorials: Lloyd and Misako commemorate Garmadon at the Corridor of Elders; Kai and Nya remember their mother and father at their blacksmith shop; Zane remembers his father in Birchwood Forest; Jay spends time with his mom and dad at their junkyard; and Wu goes to the old monastery to remember his father.
Realizing everyone is out celebrating, Ronin decides to steal from the deserted houses. Cole arrives at Yang's Temple, which is still floating in the sky. Cole confronts Yang, who he attempts to strike with the Yin Blade but misses, instead smashing it into an orb.
Consequently, the Departed Realm is opened, and the spirits of the Ninjas' old enemies escape while Cole is detained by Yang's students.
The spirits possess five of the six mannequins in the Hall of Villainy, bringing them to life. Speaking through a painting, Yang tells the villains they can remain in Ninjago by slaying the Ninja that defeated them.
Pythor then enters the museum and joins the villains as they discuss who to go after. Chen chooses to go after Kai and Nya, Pythor chooses Lloyd,Cryptor chooses Zane, Morro chooses Wu, and Kozu chooses Dareth, which leaves Samukai with facing Jay. All of them have a target but one.
"Don't look at me, I'm a past villain, I'd rather sit back and watch the fun"
"What side are you even on?" Pythor asked
"i don't have a side"
"You'll choose a side eventually" Morro said
"Hm... then i guess I'm a ninja, I am the green ninja's girlfriend after all" Aria turned around to face the other villains
"Welcome back princess" Pythor said
"I'll go join sensei Yang and Cole, that's how im going to be able to stay. Good luck guys, you'll need it" Aria said and with that the villains went their separate ways.
Cole is imprisoned in Yang's temple, where he learns of Yang's history. Yang taught his students to master Airjitzu, and around this time he found the Yin Blade, which contains enough dark magic to imbue immortality.
Yang tested the blade on himself, but its powers backfired, turning him and his students into ghosts and defacing his temple.
Yang then reveals to Cole that he plans to use the Yin Blade to tear open the Rift of Return to revive himself, while the revived villains distract the Ninja so they won't thwart his plans as Cole realizes his friends are in danger because of him.
At that moment, the villains begin their attack on the Ninja all around Ninjago. With help from Eyezor and Zugu, Chen uses his dinosaur mech to attack Kai and Nya, who flee on the Raider Bikes.
Cryptor and three Nindroids confront Zane in the snowy forest. Meanwhile, Samukai fights Jay as Krazi and Frakjaw apprehend his parents, and Pythor fights Lloyd and holds his mother captive.
Kozu and several Stone Warriors confront Dareth, who was responsible for taking control of the Stone Army.
Wu gets a bad feeling as Morro confronts him, though Wu is pained at having to fight his former student again, he states that he won't hesitate to do it again. However, Morro reveals he really has no intention of revenge; instead he tells Wu of Yang's plan and they set off to reunite the Ninja.
Back at the Temple of Airjitzu, Cole breaks free and makes his way up through the temple after escaping his captors, fighting Yang's students along the way as Yang opens the Rift of Return. He bumped into someone, he looked up as he was about to attack he stopped.
"Aria" Cole said surprised
"Hey cousin" she said
"...your a ghost" Cole said
"Yeah that's why I'm here, come on I'm here to help" Aria said pulling him and Cole smiled, they continued on.
One by one, the Ninja defeat all the revived villains, except Pythor, who flees after realizing he can't win his fight. In the meantime, Yang does Airjitzu to reach the Rift, but Cole and Aria stops him, and a fight breaks out between them.
The Ninja reunite at the museum, telling each other what happened before Wu arrives with Morro, who the Ninja prepare to attack but Wu stops them and Morro proves his innocence by unveiling Yang's plan and how the Ninja forgot Cole. Morro decided to keep the Aria part a secret.
The Ninja journey to Yang's temple while Morro's spirit peacefully returns to the Departed Realm. Cole battles Yang as the Ninja fly over on the Bounty. Yang soon overpowers Cole and mocks him as being another forgotten ghost.
"Bitch I'm right here!" Aria shouted tackling Yang
"Nice Aria" Cole said smiling
Cole's heightened vigor makes his arms rock-solid, and with them he shatters the Yin Blade. This frees Yang's students, who Cole tells to cross through the Rift and after they do they become human again.
Yang admits he failed his students and wanted to live forever so no one would forget him, to which Aria says he'll be remembered as the creator of Airjitzu.
Together, they ascend towards the Rift, but Yang tells Cole and Aria that someone has to remain in the temple as a ghost to be the "master of the house" due to the curse. Yang then decides to settle his own "debts" and launches Cole and Aria through the Rift just as it starts to close.
The shockwave of the Rift closing causes Destiny Bounty to have a hard landing at the Airjitzu temple. The Ninja soon begin to worry if Cole didn't make it in time as Jay states he'd give anything to have his friend back.
At that moment, Cole emerges from behind a rock a human again.
The Ninja celebrate the return of their friend before looking at the restored Temple of Airjitzu which Cole suggest they make into their new base. Not long after, the Ninja and their allies gather around a fire outside of Yang's temple after Cole told the story.
Jay asks him if he was going to tell the story every time the holiday comes to which Cole replies only if Jay admits he was scared the temple was still haunted to which the he states that was the case but says he was wrong as there are no more ghost.
Cole confirms this while looking back to the temple and winks at Yang, who stands inside and winks back in return. Those surrounding the fire let their lanterns float into the night sky, concluding their celebration.
"It's beautiful" they heard a voice say from behind Cole
"Cole?" Kai questioned
"I couldn't have done this alone, i did have some help" Cole admited moving to show Aria
"Aria" Lloyd ran to her and hugged her
"Lloyd move i wanna hug her" Jay whined
"No mine" Lloyd said hugging her a bit tighter and Aria laughed
"The whole team's back together again" Kai said smiling
"I'm happy to be back, i missed you guys" Aria said pulling away from Lloyd and hugging all the other ninja, all of them. And then she hugged sensei wu
"It's good to have you back" Wu smiled
"Aria you missed so much, i saved the entire team from Djinn" Jay said excitedly
"Oh did you now, i wanna hear this" Aria laughed
The ninjas all sat down with Aria sitting on Lloyd's lap.
Jay started "Ok ok so back in ninjago after the whole Preeminence thing we were famous"
And so the ninjas were all reunited again as Jay recaped what Aria missed Aria couldn't help but wonder if she was really needed.
Aria remembers that the Day of the Departed is for remembering fallen ancestors and cherishing family. So shouldn't she remember her father, he was the one who raised her after all.
Anyway all the ninja enjoyed telling the past events that Aria missed, they were all together again and are ready for whatever comes their way next. Yeah and Lloyd and Aria kissed and all the ninjas started whistling and teasing Lloyd. Jay teased Aria once and he got slapped, lessoned learned