As witnessed by the married British reporters Mike and Phyllis Watson, mysterious fiery objects falling from space into the Earth's oceans are the beginning of a series of strange events; it is eventually discovered to be the work of aliens from another world, apparently one with a much thicker atmosphere so that they feel more at home in the ocean depths and may not even be able to survive up in the air. Attempts to investigate them go badly, and it turns into a war for control of the planet.
This novel contains examples of:
- Alien Invasion: Wyndham wrote the book as a different take on this trope than the film version of The War of the Worlds which came out at the same time.
- Brand X: Mike and Phyllis work for the EBC (English Broadcasting Company). It gets extensively lampshaded — a Running Gag is that every character is introduced saying "don't you mean BBC?", and later gets subverted, when the government takes over the media and the narrator explicitly mentions that the EBC and BBC are now one and the same.
- Covers Always Lie: An at-least-they-tried example from a Penguin edition that shows an ocean liner being sunk/attacked by an alien bio-tank. Ships do sink in the novel, and there are alien bio-tanks, but they never appear in the same scene. (Also the bio-tank on the cover doesn't look anything like the tanks in the text.)
- Death of a Child: Along with mention of a dead baby being found in a crib following a bio-tank attack on the child's village, Mike notes tersely at one point that he and Phyllis had a son who died at eighteen months old.
- Deus ex Machina: Mike admits how blindly lucky he and Phyllis are to acquire a boat capable of getting them out of sunken London and down the coast to their cottage.
- Embarrassing First Name: It's not surprising that the Watsons' friend Tuny would rather be called that than "Petunia".
- Expy Coexistence: The Watsons works for the EBC (English Broadcasting Company). A Running Gag early on is that every character is introduced saying "don't you mean BBC?". (Eventually the EBC becomes well enough known that this stops.)
- Flooded Future World: Unseen aliens colonize the Earth's oceans, and eventually melt the icecaps to cripple human civilization. The bulk of the novel is set in England, and the reader witnesses London becoming mostly submerged and abandoned.
- From Bad to Worse: The aliens start with sinking ships, then send bio-tanks to steal away the populations of coastal cities, and finally melt the ice caps, disrupting most of human civilization with flooding and climate change.
- Hidden Supplies: Mike learns that Phyllis's "hobby" of bricklaying was cover for her bricking up a cellar-full of food supplies in case of disaster. "Did you really think that someone like me would be doing all that bricklaying just for fun?"
- Hostile Terraforming: Humanity sees indirect evidence that the invaders are reshaping portions of the ocean floor. They also melt the polar icecaps, but that is more likely an attempt to disrupt human civilization rather than terraforming.
- Inscrutable Aliens: A group of aliens invades the Earth's oceans. They never contact the human race in any way and the two sides engage in a war without either side ever seeing the other alive and face-to-face.
- Instantly Proven Wrong: Mike thinks that the official story about "metal fatigue" being the cause of the first alien ship-sinking will go over well with the public, only for Tuny to immediately sneer at the idea. Though as noted below, she is Right for the Wrong Reasons.
- Literary Allusion Title: A reference to the Alfred, Lord Tennyson sonnet The Kraken. In-novel, Phyllis critiques that it's not really appropriate to the situation.
- Market-Based Title: The first American edition was titled Out of the Deeps (Which as noted above, is closer to what Phyllis suggests for a title.)
- Military Mashup Machine: The aliens drive their amphibious tanks from the deepest parts of the ocean up to the coast to attack humans.
- My God, What Have I Done?: Bocker suffers a bout of this when half of his EBC-sponsored biotank expedition gets killed in an attack.
- Organic Technology: The only example of the aliens' technology seen in first-person in the story are their heavily-armored "biotanks," which are armed with jellyfish-like snaring and immobilization weaponry, used to abduct humans (alive or dead) down into the deep for some horrible fate.
- Poor Communication Kills: This compounds throughout the early parts of first contact, with tragic results. Human anti-air defenses shoot down a lot of alien craft before they fully appreciate they are alien craft, and then when they send down bathyspheres to try to examine what they're doing down there, the bathyspheres use ultrasonic sensory equipment that apparently causes them great pain, causing the aliens to lash out, killing the crew. Humans respond by atom-bombing their settlements, and it's all downhill from there.
- The Professor: Geographer Alistair Bocker, who is generally right about what's going on, and is mostly (though not universally) ignored and reviled.
- Projectile Webbing: The biotanks use jellyfish-like wads that burst into masses of sticky streamers to snare their human prey. It's noted in-universe that the strands are somehow able to distinguish between their target and nonbiological surfaces.
- Properly Paranoid: Phyllis covertly bricks up a large cache of food in the couple's seaside cottage during the early stages of the conflict, allowing them to survive later.
- "Ray of Hope" Ending: At the end of the story the invading underwater aliens have melted the polar ice caps and caused worldwide flooding. However, a more sensible and pragmatic faction has taken control of the English government, while the Japanese have developed an unmanned device that broadcasts ultrasonic waves lethal to the aliens. It has wiped out some of the aliens and might succeed in destroying all of them.
- Re-Cut: The original US version of the novel condensed a couple of sections of plot, and included a somewhat different ending.
- Red Scare: Mocked with Tuny; she continues to insist the Russians are behind the book's ever-escalating attacks on humanity from the depths of the sea, when it's soon made clear they couldn't possibly be doing it.
- Skewed Priorities: Pretty much the entire human race, which refuses to either take the ongoing threat seriously, or overreacts by throwing nukes into the ocean. A specific example is Tuny again, who grumbles about Wimbledon being cancelled even as London begins to flood and British society as a whole circles the drain.
- So Was X: Two reporters are discussing Bocker, who is claiming that the mysterious fireballs falling from the sky over the deepest parts of the ocean, the discolouration of certain currents from a spike in sediment levels and the disappearance of several bathyscapes are all a result of alien invasion. One of them points out that however crazy the idea sounds, the man does have an explanation for more of these inexplicable events, that are equally inexplicably happening simultaneously, than anyone else. His colleague has the following response: "So, undoubtedly, would Jules Verne."
- Shout-Out: Of course there are a couple of jokes about Mike's last name.
- Sudden Lack of Signal: The Watsons and the rest of the EBC's flooded London outpost abruptly and permanently lose radio contact with the new British capital in Harrogate.
- Tentacled Terror: The story is about the invasion of Earth's oceans by a race of alien cephalopods. (Or at least the organic weapons they deploy are somewhat squid-like; it's never revealed what the actual aliens look like.)
- There Are No Therapists: Averted, Mike is able to get professional help after suffering from (essentially) PTSD following the biotank attack.
- Women Are Wiser: While Mike isn't stupid, it's made pretty clear that Phyllis is the brains of the operation, with her husband mostly uncomplainingly tagging along in her wake.
