We have a three-parter here, and they're all cliffhangers — but that's okay, because this is the second book of a trilogy. To sum the three-part ending up: Pressia makes Bradwell into a monster, Lyda ends up back in the rehabilitation center, and Partridge poisons his father.
Let's explain those in a little more detail.
El Capitan, Bradwell, and Pressia are all hanging out in Ireland, drinking Guinness and eating corned beef… oh, wait. They're in Ireland, but El Capitan and Bradwell have just been attacked by some very unruly plants. Bradwell is about to die, and Pressia sprinkles some vial juice on his birdy-wings. Those wings promptly grow huge, and Bradwell resembles some sort of man-dragon hybrid.
Lyda has finally made it back in the Dome, which is drastically different than where she's been for the majority of the book (with the mothers). She's preggo with Partridge's baby, and has entered the Dome as a sort of renegade double-agent.
Partridge decides to full commit to the whole patricide thing, and gives his father a literal kiss of death: he spits a poison capsule down his daddy's throat. It looks like he's in it to win it: he's decided to lead from inside the Dome.
But you'll have to pick up Fire if you want to learn more…