Island of Lost Masks is the first book in the BIONICLE storyline of 2015-2016, followed by Revenge of the Skull Spiders and Escape from the Underworld. It retells much of the story featured in the BIONICLE 2015 online animations, with a few small additions such as naming the Protectors and featuring additional moments between them and the Toa.
A young villager named Bingzak is menaced by Skull Spiders in Okoto's Region of Jungle, but is then saved by Nilkuu, the Protector of Stone, whom he then guides to the Temple of Time. There they meet the other Protectors-Narmoto of Fire, Izotor of Ice, Vizuna of Jungle, Kivoda of Water, and Korgot of Earth-who join with Nilkuu in using the Mask of Time in a ritual to summon the Toa. With that done, they separate to return to their home regions, with Bingzak accompanying Nilkuu back to the Region of Stone. Some time later, they witness the arrival of the comets carrying the Toa, with Pohatu arriving just prior to a Skull Spider invasion of Nilkuu's village and helping to thwart it; the two then set out to find Pohatu's golden mask.
Across the island, the other Protectors and Toa do likewise: Tahu with Narmoto, Kopaka with Izotor, Lewa with Vizuna, Gali with Kivoda, and Onua with Korgot. They are eventually successful, and the Protectors and Toa then part ways as the latter make their way to the City of the Mask Makers in search of Ekimu, the Mask Maker, whose fallen brother Makuta is the source of the evil plaguing the island. The six heroes meet at the gates to the city, and initially clash while facing the threat of the Lord of Skull Spiders; however, they eventually realize the need to work together and knock him off the bridge leading into the city. Making their way into the city, they are soon menaced by other minions of Makuta's: the Skull Warriors and the vicious Skull Slicer, with the latter battle landing them buried beneath the city's surface.
Inspired by the heroism of the Protectors in their quests for the Golden Masks, the Toa find a renewed sense of purpose and return to the surface, where they face the Skull Scorpios. They then enter Ekimu's tomb and are able to revive him, only to learn that they must rush to stop the evil Kulta, the Skull Grinder from destroying his Mask of Creation. After defeating his bodyguard Skull Basher they confront him, but the villain uses the Mask of Creation to become more powerful and shatters the masks of the Toa. However, they are able to hold him off long enough for Ekimu to recreate his Hammer of Power, which he uses to knock his mask off of the Skull Grinder's face and reclaim it. He then repairs the masks of the Toa, but all know that the threat of Makuta is not ended.