The storyline of the fifteenth season directly follows on from the preceding season, Seabound, which involved the ninja character Nya becoming one with the sea and being lost to the ninja team. The first part of Crystalized focuses on the ninja attempting to save Nya by restoring her to her human form, breaking the law and becoming wanted fugitives in the process. The rest of the season centers around the return of the ninja's old enemy, the Overlord, under a new alias called the Crystal King, as he assembles some of the greatest adversaries from the ninja's past to defeat them once and for all and conquer Ninjago.