Cleo de Nile's father Ramses invites his daughters to attend a fancy gala in Boo York featuring the showing of the Comet Crystal and heralding the arrival of a comet from outer space. Cleo invites her boyfriend Deuce and classmates Frankie, Clawdeen, Draculaura, Operetta, and Catty Noir, the last of whom has been struggling to come up with a new original song. They meet aspiring performer Luna Mothews, robotic DJ Elle Eedee, and ballerina Mouscedes King. They also bump into Toralei, who was invited by Cleo's sister Nefera. Ramses and Nefera conspire to take over Boo York by trying to set up Cleo with the gala patron Lux Ptolemy's son Seth, where, if Cleo and Seth make a promise under the comet's power, the promise cannot be unbroken. Meanwhile, back at Monster High, Ghoulia discovers the comet seems to be on a collision course to Earth, and tries to stop it before it blows up the entire world.