The third book is called Locksmith's War. This is the one where, having found out everything they need to know, our friends decide to do something about it. The irony is that Lock, who has spent the last couple of books learning to communicate with and trust in others, will have to fight the final battle alone.

Tara O’Brien wasn’t sure whether this was a dark and stormy night yet, but it was definitely at least two out of three…
“All this rain — where is it coming from? Are we getting another hurricane?”
“This is no hurricane,” said Gary. “Too much lightning — not enough wind. From what I could tell, this came in from the north. It might just be random weather — we’ve only been coming here half a year, we don’t really know anything about the weather here and now in this part of the world — and there’s not a lot I can tell from one storm…” But that wasn’t going to stop him from trying. He closed his eyes and thought for a moment.
“The Arctic Ocean,” he finally said, for some reason taking care to pronounce both of the hard Cs in Arctic. “If it’s ice-free all summer — have you seen An Inconvenient Truth?”
Tara shook her head. “Why would I want to see it when I already know how the world’s going to end?”
“Well, one of the things he says is that ice reflects sunlight, but seawater absorbs it — most of it, anyway. So — this is just my own personal theory — if there’s no ice left up there, or there’s ice in the winter but it all melts in the spring, the ocean spends the whole summer under constant sunlight — it’s weak, low on the horizon, but it never stops.” Gary’s pale blue eyes lit up with the joy of explanation. “And the Arctic Ocean is surrounded by a lot of land. So the water just keeps getting warmer and warmer, a little at a time, like a pot of water under a burner set really low, until by the end of the summer it’s evaporating like crazy. There wouldn’t be any hurricanes that far north — the Coriolis force is too weak — but there’d be a huge amount of rain from it, and it’d spill all over the world as far south as… here, I guess.
“But if that’s what’s happening, then actually it’s good. With all that evaporation, the surface water would get really salty and dense and sink to the bottom, and that would get the thermohaline circulation going… again…” Gary’s voice trailed off, and his expression fell. “You didn’t actually want to hear all this, did you?”
“To be honest… when I said ‘where is all this rain coming from’ I was really just complaining about the weather. You didn’t have to answer the question.”
“I’m boring you, aren’t I?”
Tara bit her lip.
“I’m sorry,” said Gary. “I know I talk too much, it’s just… I know all this stuff, and I think of all this stuff, and I think it’s interesting and I want to share it with people, but nobody else seems to want to…” He slumped down where he sat. Tara took him by the hand.
“I’m interested in you, Gary,” she said. “You. Personally. Maybe not always in what you happen to be saying right at the moment, but…” She sighed. For most of her thirteen years of life, Gary Thalberg had been her one and only friend… And Gary was very much a package deal. Hanging out with him meant getting used to his towering intellect, his love of knowledge and his hunger to understand the universe and explain it to everybody who would hold still long enough. And were those really such terrible things?
“Oh, what the hell,” she said, resting her head on his shoulder. “Tell me all about the thermosomething circulation of the Arctic.” (She pronounced it artic, like a normal person.) “Maybe it’ll take my mind off the mess we’re in.”
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