At the end of Mortal Engines, Katherine & Bevis and Tom & Hester live happily ever after while all of London thrives and all the roaming cities live in peace and harmony.
Katherine and Bevis die painful, horrible deaths, then everyone else in London burns when the city blows sky high. And then author Philip Reeve comes along and curb-stomps a kitten in front a group of nuns with cancer. Okay, we're joking about the kittens and nuns. All the kittens blew up when London did. There are no more kittens. Or nuns.
Let's face it: the ending of this book is beyond bleak. Tom and Hester are circling London in their stolen airship as the entire town, the city Tom grew up in, lies in a smouldering heap. "Smouldering" might look nicer with that British "u" crammed in the middle, but fun spelling doesn't make it any less tragic. Hester says to Tom, "You aren't a hero, and I'm not beautiful, and we probably won't live happily ever after […] but we're alive, and together, and we're going to be all right" (37.12).
That's nice. But what do they have to live for? Each other? They didn't even like each other until just a few hours ago. However, it's not like Tom or Hester lost anyone close to them in the London explosion, so personally they're not all that affected. And they are still alive, so that's good. We guess.
Philip Reeve originally planned this as a standalone novel (source). We're glad there's a sequel, so there's some hope for these two. They can only go up from here.