We pick up more than a year later. Following the Homecoming Dance, the rest of sophomore year at Perdido High went pretty well. Austin and Cassi are dating, and are exclusive - Cassi insists that Austin is her one true love. She and Madison have drifted apart, but she's still close with Ashley. Cassi also visited Briar in the hospital after Homecoming, and they've been trying to resolve their differences. Briar and Austin have also been working to resolve their feelings...and they haven't quite managed it.
Dora has formed a coven with three other girls, all of Aztec descent. She's still dating Miguel Munoz (who is not of Aztec descent, and therefore thinks of the coven as a "girls night" kind of thing). One of the girls in the coven, Erika Perez (nicknamed "Tlatoani" for coven purposes) happens to be Cassi's cousin, and knows most of the family history, including how Cassi's father made his money. Skylar, being the creep he is, watched one of the coven meetings. Rook also attended Dora's church once over the ensuing year. Dora, for her part, wants to get back into Briar's armory - that's where her obsidian dagger came from.
Meanwhile, Genesis has been slowly learning about human things, often with Cassi as her go-to. Briar recruited Genesis to help take care of a supernatural shark (probably the one that ate Kevin) that was terrorizing the beach over the summer, and somewhere in there Genesis learned about the anonymous blog that Briar keeps, detailing her monster-hunting exploits. Genesis and Rook have continued their relationship; she also taught him to swim a bit.
Skylar has moved in with Rook and taken an after-school job at Pi, and otherwise has continued being the creepy ghost she is; she saw Cassi confront Erik about being a witch.
Rook and Cassi also have some tension; she's ditched him to hang with Austin.
So: We open in February, a few weeks before the Sadie Hawkins dance. The characters start out in Mr. Beck's home room, listening to announcements. They go about the rest of their day (and we hammer out their class schedule), and then the after-school stuff starts.
Cassi volunteers to be on the decorating committee for the dance, and Briar, in an ongoing attempt to be cool with Cassi, goes along. Her cousin Erika is also there, and they decide on "Let It Go" as a theme for the dance. Cassi is thrilled to be able to build a castle; she's in wood shop this year and everything!
Rook is thrift shopping after school. He finds a necklace, a black stone with a silver chain, in a jewelry case, and sees his reflection grab at it. He has the guy at the store open it, and finds it's heavier than it should be and the chain feels cool and smooth, like quicksilver. He buys it (along with a bunch of other stuff), and the man writes "Not over 60 degrees" on the inside of the box of the necklace.
Dora accompanies Miguel to Pi, where he works as a delivery driver. They go out on a job and he goes into an apartment complex...but doesn't come out. She goes in and finds him on the floor in front of the door, comatose and freezing. She calls 911, the door opens, and a gruff-looking man rolls his eyes and says, "I told him not to touch the doorknob." He presses a blue rose blossom into Miguel's hand and slams the door, whereupon Miguel wakes up. Dora helps him to the car (leaving the rose behind), and Miguel says that he heard the man say something beginning with "don't touch" but didn't understand the rest through the guy's accent. Then he closed the door and it felt like he'd been dropped in very cold water.
Genesis, meanwhile, is on the swim team, and is becoming the rising start (obviously; she can swim better than anyone else). While doing laps, she sees the pool water grow murky and feels something huge swim beneath her. She follows it...and bonks her head on the bottom of the pool. Dragged out of the water with her head bleeding slightly, she explains that she just got turned around. Her coach is concerned, so has her sit out of practice, keeping her out of the water (this does not make her happy).
At Pi, the characters arrive severally and talk about their weird experiences. Rook pulls out the necklace, and Briar grabs the chain. The both gaze into the abyss, and are both taken with a sudden chill (not Chill). They feel someone loom behind them, and a voice laced with frost whispers, "You're going to die."
The drop the necklace, and it crackles with rime.
Roll credits, probably with some gothy cover of "Let It Go" performed by CHVRCHES.