In 2020 the series received the Seiun Award for best manga in a tie with the manga adaptation of Batman Ninja.
The story follows a man known only as Bub, an amnesiac and the last living Earthling in the universe. Bub may not be able to remember anything about his past but he knows one thing: He absolutely wants to have sex with as many alien girls as he can.
Join Bub along with the diminutive alien Hopper and robotic driver Junk Heap as they journey across the galaxy together in search of Bub’s lost memories while encountering all sorts of strange new planets and creatures.
How Many Light-Years to Babylon? contains the following tropes:
- Absent Aliens: It turns out that humans were the only intelligent life in the universe which sent the humans of the 26th century into a state of nihilistic despair. Aliens only came into existence after the Fourth Dimension Man went back in time and decided to create them.
- Accidental Murder: Played for very dark laughs when the crew briefly visits a planet inhabited by tiny aliens. Bub ends up accidentally eating or stepping on thousands of them. Luckily the aliens are fine with dying in the name of tourists having a good time on their planet.
- Amnesiac Hero: At the start of the series Bub has zero memories. All he knows for certain is that he’s the last surviving Earthling, he’s from England, and that he has a desire to have sex with as many alien women as he can.
- Auto Erotica: Shortly after meeting, Bub and Karellen end up having sex in the back seat of the spaceship. This gets on Hopper and Junk Heap's nerves since they happen to be driving in the middle of all of this.
- Batman Can Breathe in Space: Played with. Bub can’t actually breathe in space unless the hood of his space clothes are up. Although the hood is just a hood with no breathing apparatus attached. Hopper, Junk Heap, and other aliens also never have any trouble breathing in space.
- Batman Gambit: The Fourth Dimension Man ends up having an incredibly complicated one. He destroyed the entire Earth leaving Bub as a sole survivor in the hopes that he would hook up with an Overlordian and produce children that would be able to solve the math equation that lets him become fifth dimensional.
- Been There, Shaped History: Karellen actually visited Earth in the past and gave some of her alien technology to Jesus Christ, which was how he was able to perform his miracles.
- Being God Is Hard: The Fourth Dimension Man has become incredibly bored with the universe after giving it new life and his greatest desire is to travel to new parallel universes.
- Big Bad: The Fourth Dimension Man, Abitah, ends up being this. He’s the one who destroyed Earth as part of a plan to travel to new dimensions and ends up kidnapping Akka and Vita as part of his plan.
- Bigger on the Inside: With Starbians, a species of alien, the inside of their vaginas are much larger than they look on the outside, containing what’s essentially a small living room within them. Inside these womb rooms Starbians are able to project the memories of the people inside of them.
- Boldly Coming: The initial premise of the series is about Bub's insatiable desire to have sex with alien women in order to repopulate the human race.
- Censor Box: While the manga mostly averts Barbie Doll Anatomy and shows characters with anatomically correct nipples, when Bub is naked his penis is covered with a black box.
- Character Development: Despite it being his defining character trait early on, Bub’s sex drive actually decreases as the series goes on. Once he reunites with Karellen and finds out that he did successfully have kids with her, Bub genuinely wants to try and be a father.
- Chekhov's Gag: In one chapter Bub is tossed into a relativistic prison for drunk driving a spaceship. Only three hours pass in prison while ten years pass outside. This is played off as a gag but the ten years turns out to be important later when it turns out Bub had children with Karellen.
- Child Prodigy: Despite their quiet demeanor, Akka and Vita are both quite intelligent for a pair of ten year olds. This is the reason the Fourth Dimension Man wants them to solve the Fifth Dimensional Equation.
- Creepy Twins: Akka and Vita are a downplayed example. They’re very quiet and almost always have a blank expression on their faces, but they really are just normal kids despite appearing a little odd.
- Deity of Human Origin: What the Fourth Dimension Man really is. He was originally the last human on Earth in the 26th century. But when he was transformed into the Fourth Dimension Man, he essentially became a god. He went back to the start of the universe and began populating planets with all sorts of new life.
- Ecchi: Starts out as an ecchi series about Bub going around having sex with various alien women (and sometimes men). However as the series goes on this gradually falls to the wayside as the focus shifts to Bub’s memories, his children with Karellen, and the Fourth Dimension Man.
- Easy Sex Change: Junk Heap is able to perform sex change operations very easily thanks to nanites that are able to reshape the structure of someone’s body.
- E.T. Gave Us Wi-Fi: Basically Karellen’s MO. She goes around traveling from planet to planet and giving them technology to help them develop.
- Extreme Omnisexual: Bub is comfortable having sex with most aliens out there regardless of their species, sex, or his own current sex.
- Eye of Providence: Mingo-S is designed after the Eye of Providence, being a pyramid-shaped alien with one big eye.
- Fauns and Satyrs: The true form of the Fourth Dimensional Man, Abitah, resembles a satyr.
- Given Name Reveal: "Bub" is not his real name. But after reading about the history of Earth it’s able to jog his memory enough that he remembers his real name: Daniel.
- Half-Human Hybrid: Akka and Vita are half-Human, half-Overlordian as the result of Bub’s hookup with Karellen.
- Last of His Kind: Bub is the last survivor of Earth who was found stuck inside a stasis pod by Hopper and Junk Heap one day.
- The Lost Lenore: Back on Earth Bub had met a woman on his trip to Japan and seemed to hit it off well, even promising to meet up. Unfortunately this all happened shortly before Earth was completely destroyed.
- Line-of-Sight Name: Bub gets his name because Hopper and Junk Heap found him wearing a "Sponge Bub" T-shirt and just assumed that must have been his name.
- More than Three Dimensions: The Fourth Dimension Man is an elusive being who is said to be able to travel across time and space freely. Very few people have met him, let alone seen his true form. However, he wants to become Fifth Dimensional so he can travel across the multiverse and into new dimensions.
- Nanomachines: Junk Heap is full of nanomachines that serve a variety of purposes. Most prominently him injecting Bub with them gave Bub Sex Shifter abilities and also allows him to survive being decapitated at one point.
- Occidental Otaku: What Bub was back on Earth. He’s a British man who was majorly into anime and manga, going on a trip to Japan primarily to visit Akiba.
- Only Known by Their Nickname: Everyone just calls the main character Bub. Even after he figures out his real name, Hopper and Junk Heap still call him Bub since that's what they’re used to.
- Potty Failure: Bub ends up floating through space while having to pee. Under the assumption that his space clothes will absorb and recycle urine into water, Bub intentionally wets himself only to find out that his clothes don’t work like that.
- Robot Buddy: Played with regarding Junk Heap. Despite looking like a robot, Junk Heap claims he’s actually a mechanical organism. Although Bub doesn’t quite get what the difference is.
- Reset Button: In the end, thanks to the device the Fourth Dimension Man used to destroy Earth also containing a record of absolutely everything on it, Mingo-S is able to use it to restore Earth just how it was before it was destroyed.
- Sex Shifter: Thanks to the nanites Junk Heap injected into them, Bub’s sex occasionally changes around.
- Shoddy Knockoff Product: The Fourth Dimension Man Museum is apparently unaffiliated with the actual guy and sells bootleg Fourth Dimension Man merchandise which the real Fourth Dimension Man absolutely hates.
- Shout-Out:
- Bub gets his name due to the "Sponge Bub" shirt he was wearing when Junk Heap and Hopper found him.
- Before Earth was destroyed, Bub visited the statue of Unicorn Gundam in Odaiba. This statue is indeed a real landmark in Odaiba as well.
- When Bub partially recovers his memories of Earth, one of the things he finds out about himself is that he’s a massive fan of the "Pretty Care" series. He specifically namedrops Heart Pitch Precare, Yo! Princess Precare, and Hcugtto Precare when talking to Mingo-S.
- The Fourth Dimension Man namedrops Minecraft at one point when explaining what it felt like to become a God.
- Six Degrees of Kevin Bacon: Discussed in one chapter where Bub brings up the Kevin Bacon game to Junk Heap and Hopper. They come up with a similar plan involving degrees of separation in order to find the Fourth Dimension Man.
- Starfish Aliens: Mingo-S and her species are some of the more bizarre aliens we see, being one-eyed aliens shaped like pyramids who speak in a language that very few understand.
- Third Eye: The Fourth Dimension Man has a third eye in his forehead. Bub, Akka, and Vita end up getting third eyes as well after gaining the Fourth Dimension Man’s powers.
- Twin Telepathy: Akka and Vita communicate with each other this way, very rarely speaking out loud to other people.
- Vulgar Humor: The first two chapters set the tone for what you can expect. Chapter one has Hopper’s spaceship being used as a vibrator by a giant alien woman and chapter two has Bub going inside an alien woman’s vagina which happens to have a house inside it. Although as the series goes on it tones down the vulgar comedy as the focus shifts to Bub recovering his memories.
- Weird World, Weird Food: A lot of the food Bub encounters on other planets is just plain bizarre. Such as a sushi restaurant that serves sushi on a conveyor belt so long that the food fossilizes by the time it reaches customers and Hopper’s home planet serves some kind of weird bug creature. Bub does his best not to judge on account of being British and knowing what it’s like to have his cuisine mocked.