A Darkling Plain is the fourth and final novel in the Mortal Engines Quartet series, written by British author Philip Reeve.

Whilst Wren is enjoying life as an aviator, Tom misses Hester, and has been informed by a doctor that his weak heart means he only has a few years left to live.

for the world, which seemsTo lie before us like a land of dreams,So various, so beautiful so new,Hath really neither joy, nor love, nor light,Nor certitude, nor peace, nor help for pain;And we are here as on a darkling plainSwept with confused alarms of struggle and flightWhere ignorant armies clash by night.

Six months after the events of Infernal Devices, General Naga, the now-leader of the Green Storm after the supposed demise of the Stalker Fang, has formed a truce with the Traktionstadtsgesellschaft, blossoming a new era of trade and peace.

En route, Zero's servant Rohini is revealed to be Cynthia Twite, a Green Storm spy who survived the events of Infernal Devices, and attempts to kill her, but Theo intervenes.

The pair survive the crash, but Zero is captured by air-trader Napster Varley who plans to sell her to the Traktionstadtsgesellschaft; whilst Theo is saved from slavers by Hester Shaw and the Stalker Shrike, who is unable to kill due to Zero's tampering.

In the Traction City of Peripatetiapolis, Tom Natsworthy discovers that the bullet that Nimrod Pennyroyal shot him with in Predator's Gold has damaged his heart and he has not long to live, though he doesn't tell his daughter Wren.

Tom, Wren and Wolf eventually discover a new traction town being rebuilt near the ruins, a project that was in-progress long before Magnus Crome decided to use MEDUSA.

Noticing that "New London" has no wheels, the leading Engineer Dr. Childermass explains that it uses Magnetic Levitation to float above the ground, making it less harmful to the environment.

Fishcake informs Fang that Cairo is stopping to trade with Lost Boy-ruled Brighton where they steal a limpet and make their way to the state of Shan Guo, where the Stalker will do something "important".

Walking the rest of the way by foot, Fishcake and Fang stop at a hermitage, that is inhabited by Sathya, an exiled commander of the Green Storm and an old friend to Anna.

The Anna-side of the Stalker doesn't wish to harm Sathya, and wants former London Engineer Dr. Popjoy, who resurrected her in Predator’s Gold to eliminate the Stalker-side.

Popjoy explains to her that the brain he fitted her with in Rogue's Roost was from a much older model he found in the Arctic and that it was from a "Remembering Machine" that helped to keep a nomad culture alive.

Napster Varley arrives in Airhaven, which is floating above the Traktionstadtgesellschaft, and unsuccessfully attempts to sell Zero to Wolf's father Kriegsmarschall von Kobold.

Hester, Theo and Shrike arrive at the floating city and attempt to buy her from Varley, who asks for an extortionate price which they cannot pay.

All escape in an airship heading east except Theo who stays behind to retrieve a letter that Wren sent him, but is seemingly killed by a bomb shell.

Fishcake and the Stalker Fang, whose personalities seem to be merged, steal Popjoy's air yacht and reach Erdene Tezh and her old home.

Orla Twombley, a pilot formerly from Brighton, observes the destruction and reports to von Kobold in Murnau, telling him and the remaining cities to retreat from a Green Storm weapon.

The residents of New London see the firepower of ODIN from afar, and fearing that it is a Green Storm weapon, make plans to head north when the city is finished.

Wolf hears of Manchester's destruction and is determined to continue on to London, claiming to his underlings that Harrowbarrow's creeping nature will help them survive.

Wren discovers Tom's letter and attempts to follow him with Theo, but they are stopped by Garamond, who believes they are Green Storm agents and are escaping to betray New London.

The Stalker Fang explains to Tom and Hester that she destroyed various traction cities and Green Storm bases to make the two sides fight each other, giving her time to send a command to ODIN.

Popjoy's sky-yacht fails outside Batmunkh Gompa, and Fishcake abandons Pennyroyal, who travels back to Murnau and spends time in debtors' prison before writing a truthful account of the events he survived, titling it Ignorant Armies.

When Airhaven arrives, Wren and Theo buy a new airship with Wolf's expedition money, calling it the Jenny Haniver II and become air-traders.

As the years flash by him, an oak tree grows from Hester's body, and he eventually slows this time lapse when he notices human figures nearby.

The pair take him down into a village, where Shrike finds that he was considered an old shrine statue, and the people hung flowers around his neck for luck.

When he asks if there are any traction cities in the world, the village people explain that they are thought to be fairy tales nowadays and find the idea ridiculous.

Shrike starts by speaking the first lines of Mortal Engines: "It was a dark, blustery day in spring, and the City of London was chasing a small mining town across the dried-up bed of the old North Sea..."