April and her fellow, not-normal classmates at Winterborne House -- Sadie, Tim, Violet, and Colin -- thought they'd sussed out who that was: Gabriel Winterborne, their unseen benefactor back-from-the-sea and incorrectly presumed dead. They even prevented a coup of Winterborne by a distant cousin who mistakenly thought he'd killed Gabriel as a child.
But in the process, they lost the love of Gabriel's life -- Isabella Nelson, and the administrator of the Winterborne foundation. She fell into the sea, and three months later, she's still not back. She's alive, they know, because she's sending postcards that say: "STILL MAD." In all caps.
Then CPS (Child Protective Services) shows up and of course raises all sorts of alarm bells on how the children are being raised by the PTSD-suffering, highly-unreliable Gabriel, without Izzy -- who previously took care of all the "kid-student" stuff. The paperwork. The schooling. Ordering clothes in the right sizes. Making pancakes. Buying the groceries for the pancakes. Basically everything.
The specter of being taken away from Winterborne is established. Re-established? And then the kids follow Gabriel on what they think is one of his Sentinel rounds and discover, he's been lured by the promise of Izzy's presence to a fundraiser to rebuild the burned down museum attended by…dozens and dozens of Sentinels.
In a room full of Sentinels, one man uncannily knows who Gabriel is under his Sentinel disguise. When they get home, a Sentinel is there and fights with Gabriel, wounding him. Badly.
The next morning, there's a large pool of blood and he's gone. Then the CPS investigator rings the doorbell.
And if I say any more, I'll ruin the read. But just know, there's a plot afoot and a new villain / enemy who's already launched the first salvo in the fight for Winterborne. Enjoy!