Set initially in the same universe as The Moon Is a Harsh Mistress, The Cat Who Walks Through Walls is one of Robert A. Heinlein's later works. It features yet another soldier-of-fortune in the character of Richard Ames, a retired colonel who begins the story living on a cushy space station. His easy life is quickly upended when he marries Gwen Novak, who is secretly a Time Agent, and on a mission to save the universe. Richard and Gwen are chased across the galaxy by unknown enemies, and eventually take refuge in a multiverse paradise, populated by many familiar characters from other Heinlein books, including Jubal Harshaw and the ubiquitous Lazarus Long, the latter of whom tries to recruit Richard into his Time Police. Lazarus' ultimate goal is to rescue Mycroft Holmes, a dormant supercomputer with the power to perfectly predict the consequences of time manipulations.The story operates heavily on the ideas perpetuated in Time Enough for Love and The Number of the Beast — The World as Myth, Heinlein's personal philosophies regarding group marriages and the perils of socialism. It also features one of the most confusing endings of any of Heinlein's novels, leaving two characters stranded and alone and refusing to say whether or not anything was actually achieved. In fact, the resolution to the Cliffhanger ending is not revealed until the sequel, To Sail Beyond the Sunset.Despite the title, this is definitely not a part of The Cat Who... Series.
This novel provides examples of:
Achievements in Ignorance: Pixel, the titular cat, can walk through walls and possibly through space-time-universe. At a loss to explain how, the characters theorize that he doesn't realize that he can't.
Action Girl: Despite Richard's attempts at machismo, Gwen is a far superior combatant.
Almighty Mom: "Auntie" Washington, who springs Richard and Gwen from Hong Kong Luna through sheer force of personality.
Apocalyptic Log: The story is written in first person and framed as Richard writing his memoirs. The final Cliffhanger is written as a recovered journal recording, without revealing to the reader whether Richard survives or not.
Author Filibuster: Heinlein is (in)famous for this. This book is not quite as bad as others, but there are a few notable instances, particularly when the main character is shouting or sneering at bureaucracy.
Author Appeal: All of Heinlein's favorites make an appearance—group marriages, free love, TAANSTAFL, witty (and somewhat sexist) innuendo. This book practically runs on Author Appeal.
Bavarian Fire Drill: Gwen's misdirection of a customs agent starts with claiming she has a baby alligator in her purse and ends with the two exchanging recipes.
Bolivian Army Ending: Richard and Gwen are stranded and apparently left for dead, wounded and nearly out of ammo, and expecting the return of their enemies at any second.
Canon Welding: An integral part of The World as Myth concept, this novel brings together heroes from every other timeline Heinlein wrote in the effort to rescue Mycroft Holmes from Luna.
Casual Interstellar Travel: The World As Myth has been taken to its logical conclusion in this novel, with the Burroughs device enabling instantaneous travel from any point in space-time-universe to any other, and thereby making war possible on a scale never seen before.
Chekhov's Gunman: Pixel, the titular cat. He walks not only through walls but also space-time-universe, apparently via not being aware that he can't. In the climax, he alerts the heroes to an ambush by showing up at an opportune moment.
The Chessmaster: Lazarus, and Gwen by proxy. Their scheme to manipulate Richard is epic in its scope, and that itself is only a small part of the larger plan which is to rescue Mike Mycroft from Luna.
Cosmic Retcon: By way of persuading Richard to join his organization, Lazarus arranges for a particular unsavory incident from Richard's past to have never happened. This is of course to demonstrate the power of the Burroughs device to literally rewrite histories on a whim.
Easy Amnesia: Tertius medicine seldom employs anesthetic drugs; instead they engage a field that interrupts storage of short term memories. You can be awake and alert while things are done to you, but won't remember it afterward.
Eat the Dog: When passing through Lunar customs, Gwen claims to keep a baby alligator in her purse and declares it to the agent as a pet and possibly food. She is lying, of course; what she really wants is to avoid having the purse searched.
Every Man Has His Price: Richard and Gwen bribe their way through Luna, where this seems to be standard operating practice. Richard himself appears incorruptible but succumbs to a more indirect form of bribery: having his past rewritten.
Free-Love Future: On some planets, and most especially Tertius. Richard, who comes from relatively free Luna, is shocked by the level of perceived promiscuity on Tertius, but being a Heinlein character, he quickly adapts.
I Know You Know I Know: Lazarus and his enemies at times seem to be playing this sort of game, in a Sherlock and Moriarty fashion, over who can be more successful at making their manipulations stick. At one point, an entire planet is nova bombed by their adversaries, after which his team of genius mathematicians figures out a scheme to rescue all its people just prior to the event, so that the attack appeared to be successful but nobody was actually lost.
It Is Pronounced Tro-PAY: A rather involved discussion takes place regarding the cultural ramifications of the spelling and pronunciation of the name Tolliver/Talliafero. It turns out to be a Red Herring anyway.
Kinky Spanking: Young Gretchen offers to let Richard spank her as a blatant sexual proposal. He turns her down, telling her to grow up first. Thanks to Time Travel, she does, and reinstates the offer. Richard also repeatedly threatens to do this to Gwen, but we never see him carry it out.
Left Hanging: Some of the characters at the end. This is in part to expand on the concept of characters being written by an eternal and possibly ambivalent Author.
Subverted in that Gretchen is pregnant with Richard's child and he hasn't had sex with her... yet. So we know he survives.
Limb-Sensation Fascination: Richard gets a replacement for the foot he lost in the war. The first thing he does is faint. The second is spend a day reliving the marvel of having two fully functional feet.
Love Interest: Gwen gets Richard to marry her in the first chapter, invoking this trope as a means of ensuring that he'll have a reason to do what she wants, while making him think it's all his idea.
Luke, I Am Your Father: The typical formula is varied here by having the son be told by a third party after figuring out that his donated cloned-tissue foot belongs to Lazarus (because Lazarus is walking around barefoot, like everyone else). Lazarus being his father (thanks to Time Travel) is why Richard's body didn't reject the foot from Lazarus's clone.
Obstructive Bureaucrat: The Manager of the space station where Richard lives evicts him in a passive-aggressive manner by turning off his lights, changing his locks, and refusing to acknowledge that any such was done. Gwen gets even by smearing Limburger cheese on the heating element in the Manager's office.
Perspective Flip: In one alternate universe, Albert Einstein is seen as a villain worse than even Adolf Hitler, because he is blamed for the invention of nuclear weapons.
Precocious Crush: Gretchen on Luna develops a crush on Richard. She enlists in the Time Corps and ages up in a different timeline, just so she can bundle with Richard when she gets back — a few months later from his perspective.
Ret Gone: One of the members of Lazarus' Time Police force is a Lensman. After he shoots Richard in a fit of anger, they arrange to have him removed from existence. The effect is described as like seeing him rubbed out by an invisible eraser, followed immediately by the mortal wound that he inflicted vanishing.
Secondhand Storytelling: The Final Battle against the rival time force is not narrated at all, but recounted via Richard's dictated journal recording, which we are supposed to think is posthumous.
Shoot the Shaggy Dog: At the end, Gwen and Richard are both wounded (Gwen severely), and have been left behind in the retreat from Mike's mainframe. They have no way of knowing if the mission was successful, and are certain to die if their enemies return to finish them off. Part of Richard's Rage Against the Author has to do with this.
Staged Shooting: Richard thinks that this has happened and that he's being framed for a fake murder. It turns out later to have been real.
Time Police: Lazarus has founded an organization of Multiversal sentinels whose mandate is to protect the integrity of history across multiple universes.
Time Travel: The massive overlapping use of this throughout the novel leaves the protagonist hopelessly confused at times, and by proxy, the readers.
Time Travel Romance: Played so casually as to be almost flippant, given the Burroughs device's capability to render time no more of a barrier to a relationship than walking a few yards. The most obvious examples include Gwen with Richard, Gretchen with Richard, and Lazarus with Richard's mother.