With power over demons, Valentine could summon all the demons an evil overlord could ever possibly need.
He needs the blood of a warlock, a vampire, a werewolf, and a faerie in order to do it. And all these creatures have to be children.
Clary accidentally spills hot coffee on her hand. Simon kisses it better—in front of Jace, Clary's brother/lover—and Clary is uncomfortable.
Alec gets a phone call from Isabelle, who says that the Queen of Faerie has summoned them to the Seelie Court.
He thinks it's a good idea for them to go, because she might ally with them against Valentine.
It'll be dangerous because faeries "engage in creative truth-telling" (2.8.154) and are "as cunning as snakes" (2.8.154).
Because of the danger, everyone decides to go—Jace, Clary, Simon, Alec, and Isabelle.
They meet at Turtle Pond in Central Park, and Isabelle leads them into the pond.
The doorway turns out to be the reflection of the full moon on the water. It's a portal that sucks them underground.
Once everyone lands underground, a faerie named Meliorn comes to fetch them. It seems he and Isabelle may have dated before. But Jace still doesn't trust them.
He leads them to the Seelie Court, but there are lots of temptations along the way. Jace warns Clary not to dance with any faeries. "If you dance with them […] you'll dance until you die" (2.8.273). Dance, dance 'til you're dead!
Isabelle blindfolds Simon and leads him so that he won't be tempted.
Finally, they reach the Seelie Court, and the Queen welcomes them.
She offers them food and drink, but Jace tells them not to eat or drink anything while they're there.
They tell the Queen that Valentine stole the Soul-Sword, and that he will use it to turn demons against all the Downworlders.
While Jace and the Queen talk, a tiny sprite bites Clary's finger. She sucks the blood off the tip of her finger.
The Queen is surprised Jace would betray his own father. She calls him and Clary "Valentine's little experiments" (2.8.341) and tells Jace to ask Valentine what she means the next time he sees him.
They rise to leave, but the Queen says Clary must stay. She drank the faerie drink.
She swears she didn't, but it turns out she had some on the tip of her finger when the sprite bit her.
Clary wonders why the Queen wants her. "I don't know anything about Shadowhunting," she says (2.8.361).
The Queen tells Clary that she has a special gift unlike any other, "the gift of words that cannot be spoken" (2.8.364).
The Queen says that Clary can be freed with a kiss, so Simon smooches Clary.
It doesn't work. The Queen says only "the kiss that she most desires" (28.388) will break the spell. Sorry, Simon.