It is set in a fantasy world that resembles Japanese-occupied China in the 1930s. The dominant power in the setting is the Kou Empire, a Fantasy Counterpart Culture of Imperial Japan, who are launching military raids into a number of neighboring countries collectively called "the Continents." Another key feature of this setting is the Sky Container, a mysterious device that allows anything equipped with it to fly, even objects as large as battleships.
The story follows a teenage boy named Gaw who is the last living member of his people. He is under a curse, which caused him to lose his arm. Now seeking out a magician to break the curse, he meets a girl named Fa. She, too, is under a curse—anyone who touches her will die. After Gaw saves Fa from being kidnapped and forced to be a prostitute, the two of them search for the ancestral home of Gaw's people, in hopes that the key to lifting their curses might be there. There, they meet up with a group of former prisoners of the Kou Empire, and their fight for survival truly begins.
Futago no Teikoku contains examples of the following tropes:
- Airborne Aircraft Carrier: Many of the flying warships are equipped with catapults to launch fighter planes, and cranes to retrieve them out of the air. In real life, regular sea-going warships were also equipped with such devices before helicopters made them obsolete.
- Alternate History: Albeit one that still resembles ours pretty closely. The biggest difference is the flying ships and Sky Containers.
- Always Identical Twins: A pair of identical twins named Yoyu and Zoyu show up in chapter 5.
- They used to be identical triplets, but then one of them got killed by a Kou soldier.
- Amazon Brigade: The crew of the flying ship Unebi
- Applied Phlebotinum : The Sky Containers, which Gaw's people invented.
- Bestiality Is Depraved: Implied, but thankfully never shown. It's said that the crew of the Unebi consists of fifty former sex slaves. But when we see them, they turn out to be forty-nine women. . . and one dog.
- Big Damn Heroes: Gaw and Fa try to escape a Kou Navy bombardment on Gaw's glider, but are cornered by a Kou fighter plane. Fortunately, an airplane piloted by a woman named Paula intervenes, smashing directly into the fighter.
- Black-and-Grey Morality: The protagonists are pretty unlikable, especially Gaw, who repeatedly makes sexist remarks toward Fa. But the Kou Empire is a whole lot worse.
- Can't Have Sex, Ever:
Brought up when the crew of the Unebi discusses that Fa is unable to touch someone without killing them. - Cassandra Truth: Fa warns the human traffickers not to touch her, or bad things will happen to them. They assume she's trying to defend herself and not them.
- Celibate Hero: Gaw is still a virgin and does not care to change that, even when four women fight over the opportunity and his coworkers offer to find him a working woman. He claims that he is trying to cure Fa's curse merely so that he can take her, but when he has to undress her for a medical exam, he has to be coerced into it and is somewhat horrified to do so. The same goes for any other sort of nudity.
- Central Theme: No group is monolithic. The Continentals are comprised of too many different ethnic groups and social classes to judge as one, the Wufang copes with their past in a variety of ways that contradict the others and even the Kou Empire has a wide variety of morality between its soldiers.
- Cool Boat: The flying warships.
- Cool Plane: A variety of aircraft appear in the story, some real and some fictional. The most notable one is Gaw's glider, which is stored inside a Sky Container.
- Damsel in Distress: Subverted by Fa. When we first see her, she's been kidnapped. Then her curse kicks in, and her kidnappers die from touching her.
- Diesel Punk: As a series with flying battleships, it definitely qualifies as either this or a very advanced form of Steampunk.
- Doomed Hometown: Gaw's hometown was wiped out by the Kou Empire.
- Drinking Contest: The Captain of the Wufang wins The Unebi in one and the Kong Xia are notorious for winning these as well. In the Continent, everyone seems to have these to pass the time rather than win anything.
- Driven to Suicide: One of the other girls abducted by Fa's kidnappers hangs herself.
- Emotionless Girl: Fa, at least at first.
- The Empire: The Kou Empire, obviously. Gaw's people used to be one before they were exterminated.
- Fantasy Counterpart Culture: The Kou Empire is this to Imperial Japan. The Continents are one to China.
- Fantasy Gun Control: Averted.
- Gaslamp Fantasy: Combines World War II-era technology with people who have magical powers.
- Greater-Scope Villain: The Kou Emperor.
- Hammerspace: The ability of the Sky Containers to transport and store large objects is represented this way.
- In Spite of a Nail: Aside from the Applied Phlebotinum, the setting's history seems remarkably similar to our own. There's even mention of the 1922 Washington Naval Treaty, and the blurb on the cover says it takes place in Japanese Imperial Year 2595, or 1935. In volume 2 it's revealed that the Koumintang (the Chinese government led by Chiang Kai-Shek, which would later go on to form Taiwan) still exists.
- Katanas of the Rising Sun: To be expected, given the subject matter.
- La Résistance: The crew of the Unebi, all of whom are former Kou Empire prisoners.
- Last of His Kind: Gaw is introduced as this. It turns out not to be the case.
- Left Hanging: The series was cut off on a cliffhanger, leaving the fate of Yoyu and Zozyu unknown after they were captured.
- Lighter and Softer: By Mohiro Kitoh standards, anyway. It lacks the constant nihilism and visible underage sex of Shadow Star and Bokurano, and while it's still extremely violent, there is little visible blood to be seen.
- Mukokuseki: The characters from the Kou Empire (the story's equivalent of Japan) are drawn this way, but the ones from the Continents are given more recognizable east Asian features.
- Meaningful Rename: The Wufang members no longer go by their original names, choosing to cast aside their past and their original names. Fa has been spared so far, but Gaw was given the moniker "Gou Bin" to teasingly imply he's not well endowed.
- Politically Correct History: Averted. The story goes out of its way to show Imperial Japan at its worst. Subjects like "comfort women", institutionalized racism, and the extermination of conquered people are put on full display. This approach is both unusual and refreshing for a manga about World War II.
- Politically Incorrect Hero: The Kou Empire definitely aren't nice to their subjects, but if Gaw's occasional sexist statements are any indication, neither were his people.
- Ragtag Bunchof Misfits: Explored with the Wufang. They are composed of escaped comfort women taken by the Kou Empire and hail from many different backgrounds, don't always get along especially once Gaw forms an arrangement with them. They have proper military training and access to military gear, but these all come from the side that's clearly losing against an empire that largely knows how to handle people like them, even if it can't win every battle.
- Rei Ayanami Expy: Fa has several features of one; she has short, blueish hair, is very reserved and stoic, has a strong attachment to Gaw, and has a magical curse placed on her.
- Revenge Before Reason: Yoyu and Zoyu go on an impromptu mission almost guaranteed to fail to kill the Kou General that took them and their sister as his personal comfort women. They did zero planning and their mission needlessly endangered the 8 year old, dog and airplane that they brought along with them. Justified, as seeing the familiar platoon nearby triggered them to do something so drastic.
- Shown Their Work: As is usual for Kitoh's manga, there is great attention to technical detail in the depiction of aircraft, as well as the historical aspects of Japan in the 1930s.
- Sex Slave: Very common in the Kou Empire. Fa was saved from becoming one, and in the most recent chapter Yoyu and Zoyu are captured to be sold into sex slavery.
- Stern Chase: The heroes are constantly being pursued by the Kou military.
- Team Mom: Paula serves this function to Yoyu and Zoyu, and to the Unebi crew as a whole.
- Team Pet: The Unebi crew has this in the form of Unan, a French Bulldog.
- Teeth-Clenched Teamwork: Gaw joins forces with the Wufang despite a near majority wishing to permanently avoid men and Gaw having no concept of women as anything other than slaves. Its a purely pragmatic arrangement, but he has been loyal so far.
- Undressing the Unconscious: Gaw must do this to Fa so that she can receive medical treatment. Gaw absolutely does not want to do this and Fa for once shows horror at his actions, even if she understands why he had to.
- Unwanted Harem: Played with hilariously. After Gaw saves Zoyu and Soyu et al. from their poorly thought out suicide mission, three members of the Wufang want to "repay" him for his bravery and start fighting with his slave Fa over the rights. Gaw merely avoids them to take a nap, then has Fa stand guard to prevent any funny business, but allows the 8 year old of the crew to take a nap with him because she's not going to try anything.
- Virgin-Shaming: Once the inhabitants of the refugee village Gaw and Fa visit discover that Fa kills anyone who touches her, they tease Gaw about not getting laid.
- War Was Beginning: The story takes place near the beginning of the equivalent of the Second Sino-Japanese War.
