Mask of the Sensei is a LEGO Ninjago: Masters of Spinjitzu book that was released in February 2012. The events of the graphic novel are set soon after the events of the pilot episodes and before "Rise of the Snakes."
At the Four Weapons Blacksmith shop, Kai and Nya work on making weapons, only for Kai's katana to end up being brittle and easy to shatter. Before they can continue, however, they're interrupted by a panicked villager, who informs them that an elderly man had been hit by a cart and knocked unconscious. To the siblings' horror, they realize the man is none other than Sensei Wu.
While tending to Sensei Wu in his bed, Kai and Nya react with joy when he wakes up, only for Wu to proclaim that in order to protect Ninjago from Lord Garmadon, they need to conquer the continent. Shocked, Kai and Nya come to the conclusion that the blow to his head had damaged his mindset.
In order to start off his conquest, Wu destructively attempts to collect his self-declared 'Ninja Tax' from the villagers of Ignacia, angering them. However, Wu manages to rally the villagers under his control by promising to convert the village into the capital of his empire, making them all rich. Even as Wu goes off, Kai confronts him over his promise, only to learn Wu plans to destroy his home village in order to set an example. Enraged, Kai attacks Wu, only to be soundly defeated.
Meanwhile, Nya brings Cole, Jay, and Zane to help her reason with Wu. The four of them decided to humor his demands before they can figure out a plan of action, but before they take their leave, Cole asks Wu if he wants tea, only for Wu to express disgust over the idea. Realizing their sensei must be an impostor, they decide to split up and find the items the Impostor Wu has tasked them to find; a scale from an angry Dragon, dust from a river, a snowball from the desert, and a scroll from the safest place in Ninjago.
In his cave prison, Kai manages to recover from his defeat and discovers the identity of his fellow prisoner, the real Sensei Wu. Wu proceeds to reveal that the Skulkin had originally been tasked with keeping several horrible beings trapped in the Underworld, but when Garmadon got them to try to steal the Golden Weapons, they neglected their duties, allowing some of their prisoners to escape.
Jay attempts to get a scale from Wisp by angering him, eventually resorting to insulting Wisp's ability to fire lightning. Succeeding, he retrieves one of Wisp's scales and manages to calm him down after he lands in a mud puddle, causing Wisp to laugh in response. Meanwhile, Cole realizes 'dust in a river' is actually gold dust, and attempts to negotiate his retrieval of a villager's pan of dust. Despite the villager's accusations of him being a thief, Cole manages to calm him down and borrows the gold dust, promising to return it.
Later, Zane, while traveling the desert, discovers a nearby mountain with an icy peak full of snow. Formulating a plan, he forms sleds and at the top of the peak throws a snowball out into the desert. Sliding down the side of the mountain, he arrives in the desert and retrieves the snowball from the sand before freezing it with the Shurikens of Ice, preserving it.
Meanwhile, Wu reveals to the true identity of his impostor to Kai; a strange, smoke-like creature so ancient that its species' name was forgotten. Armed with the ability to shape-shift into beings and objects from the memories of others, the Smoke Creature had used Garmadon's memories to shapeshift into Wu, being inspired by the latter's dreams of conquest.
Realizing that by pretending to have been hit by a cart, the Smoke Creature could blame any mistakes that he made as Wu by using the supposed brain damage he suffered, Kai is told that thankfully, the Smoke Creature's species can only stay in Ninjago for a week before dissipating and being dragged into the Underworld. However, Wu reveals that four items can severe this connection; A scale from an angry dragon, dust from a river, a snowball from the desert, and a scroll from the safest place in Ninjago, prompting Kai to ironically remark that no one would be stupid enough to look for those items.
Jay, Cole, and Zane reunite with their respective items, discussing what might be the safest place in Ninjago, unable to figure it out. Meanwhile, the Smoke Creature wonders to itself the same question, before realizing the answer. Back in the cave, Kai approaches a meditating Sensei Wu, revealing he's figured out a plan but needs a scroll to help. After being given Wu's grocery list, he reveals his identity as the Smoke Creature, only to be attacked from behind by Kai, the two of them having anticipated the trick.
Admitting his respect, the Smoke Creature taunts the two of them by shape-shifting into Samukai, Garmadon, Cole, Wu, and Nya, before deciding to get rid of them and turns into Flame. However, Cole, Jay, and Zane arrive to help, having figured out that the passage to the prison was placed in the Four Weapons shop. Under Wu's orders, Zane uses the Shurikens of Ice to create an icy mirror, and the Smoke Creature, noticing his reflection, reacts violently and in horror.
Even as they escape the collapsing prison, Wu reveals that the weakness of the Smoke Creatures is that they can't stand their own reflections. Meanwhile, Nya arrives in Ignacia with a special artifact in tow; a gong that, when rung, lets the user hear whatever a loved one hears. Using the gong, she exposes the Smoke Creature, snapping the villagers out of their manipulation.
In the desert, the ninja and Sensei Wu gather to confront the Smoke Creature, where it attacks them in the form of a colossal crab. However, when the ninja threaten to destroy the items it needs, it attempts to negotiate, only for Sensei Wu to see through its lies. Angered, it reveals it had escaped the Underworld with four others of its species, and after shape-shifting into their opponents, battle their respective ninja counterparts.
Kai eventually manages to defeat the Smoke Creatures by using Spinjitzu to open an aquifer, the resulting water spout creating several reflective puddles. As a result of their time outside of the Underworld and seeing their own reflections, the Smoke Creatures dissipate and are dragged back into their prison, and the ninja go off to drink tea and celebrate.
Back in their home village after the battle with Garmadon, the four ninja are hoping for some R&R. Instead, they get drawn into a fight to the finish with the most powerful and unexpected foe they have ever faced: Sensei Wu—their own sensei! Can the students hope to defeat the master?
Way of the Ninja · Masters of Spinjitzu · The Golden Weapons · Rise of the Snakes · A Ninja's Path · Pirates Vs. Ninja (Book) · The Green Ninja (Book) · Attack of the Nindroids · Techno Strike! · The Titanium Ninja (Book) · The Rescue Mission · Ninja vs. Ninja · Spy vs. Spy · The Quest for the Crystal · Return of the Djinn
The Challenge of Samukai · Mask of the Sensei · Rise of the Serpentine · Tomb of the Fangpyre · Kingdom of the Snakes · Warriors of Stone · Stone Cold · Destiny of Doom · Night of the Nindroids · Who is The Phantom Ninja? · Comet Crisis