In the world of Mortal Engines, towns are always on the move—and not in the way that New York City is always on the move (well, except for the traffic). These towns literally move. They've been put on giant tank treads and trundle around the landscape like giant fortresses. The large towns pursue the small ones, and eat them. It's a town-eat-town world, baby, and we just read about it.
Young Tom Natsworthy's adventure starts when a hooded assassin attacks his hero, Thaddeus Valentine. Desperately wanting to be a hero, Tom gives chase. The assassin turns out to be a hideously, grotesquely ugly girl—seriously, Tom goes on and on about how ugly this girl is. Her name is Hester Shaw. Before she jumps to her death, she tells Tom that Valentine is the one who disfigured her. When Valentine finds out that Tom has seen Hester, he pushes him down the waste chute to his death.
Tom survives, and he and Hester the Ugly begin a long trek back to London. Along the way, they encounter sky pirates, air pirates (no Jack Sparrow-type sea pirates, though), and slave traders. Oh, my.
Because things aren't rough enough for the young duo, they're also pursued by a cyborg Stalker named Grike. Grike's mission: to kill Tom and the ugly girl and bring them back to Magnus Crome, Lord Mayor of London. Hester and Grike have a history, however, and Grike plans to make Hester a Stalker like him. She's down with that plan, but Tom isn't. He kills Grike.
Back in London, Valentine's hot daughter, Katherine, is trying to figure out what her father's assassination attempt was really about. She thinks the whole thing is pretty suspicious, and it gets even fishier when Valentine is sent away on a secret mission. She teams up with a handsome Engineer boy, Bevis Pod, and they investigate something called MEDUSA.
MEDUSA isn't a snake-haired gorgon... it's worse. Magnus Crome uses MEDUSA to annihilate a nearby town, showing everyone the power of this deadly weapon. He plans to use it to blow up the Shield-Wall and devour the stationary city of Batmunkh Gompa. To assist in this plot, Valentine infiltrates the city to disable their air fleet. Conveniently, Tom, Hester, and their friend, Anna Fang, are there. Anna Fang and Valentine engage in a clashing sword duel, but Valentine triumphs. Tom and Hester take Anna Fang's airship and race back to London, where Katherine and Bevis are planning to use a homemade bomb (a valuable skill for any fifteen-year-old) to blow up MEDUSA.
All these plot threads come together and get tangled up in a big ol' knot. Valentine tries to kill Hester (who just might be Katherine's half-sister), but Katherine jumps between them and is slain by her own father. MEDUSA is stopped when a dying Katherine plops down on the keyboard. As MEDUSA short-circuits, Tom and Hester fly away from London, and it blows up behind them. Yikes. So, pretty much every character in this book is dead, except Tom and Hester. They're alone, but alive. Happily ever after, or crappily ever after? You be the judge.