The novel opens with a short prologue in which Amane, age 20, is in bed with her boyfriend, who comments that they are the opposite of Adam and Eve. Adam and Eve were expelled from Paradise after eating the forbidden fruit. Now people are returning to Paradise and as the last couple to have sex they are the opposite of the original Adam and Eve. He thinks of her as the last Eve, which she feels is a curse.
Part 1
Amane grew up immersed in her mother’s world, surrounded by photos of a father she never met, raised on tales of her parents’ romance and how she had been born out of their love for each other. Her mother continually impresses on her the importance of falling in love and having children. However, when she is ten years old she learns in a sex-education class at school that this is now contrary to the norm. Some boys find out that she was conceived in an “incestuous” relationship between her parents and start bullying her. She feels that her mother has placed a curse on her. Amane’s first love is an anime character called Lapis, a 7,000 year-old immortal with the appearance of a 14-year-old boy. She begins to feel sexually aroused when watching DVDs of him, and she carries a picture of him in her pass case, just like all the other girls do of their crushes. One day she has an orgasm, and feels that she has had sex with him. The next day she gets her period, and her mother takes her to hospital to be fitted with a contraceptive device as per the norm. She discovers that a boy in her class, Mizuuchi, is also in love with Lapis, but doesn’t want anyone to know. They become close friends, and he confesses he wants to be a doctor and research artificial wombs to enable men to be able to have children without the need for a family. Amane persuades him to have sex as an offering to Lapis. She never feels sexually aroused with him, but she finds this reassuring as she fears feeling the kind of love that her mother, who physically disgusts her, has continually impressed on her as being important. She wants to discover her own true instincts and sexuality, not what her mother has told her. She subsequently has a string of lovers, both fictional and people. As always with Murata, sex is described in purely physical terms and is not titillating in any way. After college she gets a humdrum office job and, deciding she wants children, she marries a man she meets at a singles matching party. However, that marriage is annulled after he sexually assaults her, and some years later she marries Saku after meeting him at another matching event. They decide that Amane will be artificially inseminated once she turns 35, and start saving to buy a house.
Part 2
Amane and Saku are now three years into their marriage and happily settled. They both have lovers, and regularly confide in each other: Saku has a real life lover, but their relation is fraught and he finds refuge in his marriage; Amane continues to have fictional lovers, but always longs to feel them physically.
The word “family” comes up a lot in their conversations, it resonates almost like a prayer and every time they say it they become ever more fervent believers. In contrast, many of her friends feel they should be able to marry their best friend rather than a stranger. Amane’s co-worker Ami goes further by saying that she is saving up enough to take a few years off work to have a baby. She doesn’t want a partner, and is glad she has a womb. This makes Amane wonder whether Saku would feel the same way if he had a womb. Does he only consider her family because she has a womb? Meanwhile she meets Akitsu, a happily married man living in the same condo, and they start a relationship. Akitsu has had many lovers, but has never had sex with any of them—he now tries it at her urging.
One day Amane and Saku watch a TV program about Eden, now celebrating its tenth anniversary. Every year on December 24, a computer selects which residents will receive artificial insemination, numbers are controlled to ensure the population remains stable. Men can have an artificial womb attached—so far none has managed to carry the fetus to term, although there have been successful implants so it is only a matter of time. All newborn babies are taken to the Center, where they are raised until the age of 15, when they are considered adults and can themselves be impregnated. All the residents are considered mothers of all the children. The children, the eldest of which are now 8 years old, are more advanced mentally and physically than those raised by families. The system ensures absolute equality and is considered paradise, hence its name. If successful, it will eventually replace the family system, and humans will be released from the effects of the forbidden fruit and will be able to return to paradise. Not long afterwards Akitsu splits up with Amane saying he simply dislikes sex, and soon afterwards Saku’s lover attempts suicide: this is the catalyst for the couple to move to Eden. They dissolve their marriage, commit to a two-year resident permit, and, holding hands, get the train to Chiba and go through immigration at Narita to their new life.
Part 3
Amane and Saku now have two adjacent 7th-floor apartments, one of which they use as their bedroom, the other as their living room. They are determined to secretly continue living as family, although it is forbidden here. Their landlord takes them to the park to meet the children, who all have identical “pudding bowl” haircuts and wear identical white smocks. They call all the adults “mother” and adults are encouraged to shower them with affection. The staff in charge are more like elder siblings. The children have different features, but they all have identical expressions and gestures, as do the staff. It is like a factory to produce “convenient people.” Saku enjoys it, but Amane feels a chill, thinking that it’s like the whole town is keeping human kids as pets. Amane and Saku both take jobs, and in December are both called up for insemination. Amane’s ex-boyfriend Mizuuchi is now a doctor here, and he helps them cheat the system to ensure that Amane is impregnated with Saku’s sperm. Saku for his part is fitted with an artificial womb and implanted with Amane’s egg. A month later, however, Amane has a miscarriage, which saddens her. By now Saku and Amane are growing apart. When he goes into hospital for monitoring of his pregnancy she visits daily, although she can’t help feeling he is like an experimental animal and the baby a parasite. He becomes the first man to take his pregnancy in the artificial womb to term successfully, and she watches the operation of his baby being born but is then unable to visit him again until two weeks later once the media furor is beginning to settle a little. By this time he has already sent their baby to the Center without having consulted her. She reminds him that they promised to bring the child up together, but he tells her that they were wrong—all children belong to humankind. Amane goes to the place where all newborns are taken and a nurse allows her to take a look through the glass: the rows of babies look like a vast human cabbage patch. She can no longer tell which is their child. Amane’s mother comes to visit her. Saku by now has moved out, and Amane drugs her and locks her in his old apartment, keeping her as a pet. Some years later we learn that Amane has been artificially inseminated a number of times, but has never managed to take the pregnancy to term. One day she goes home to find a child outside her door—it must be from the first round, now 14 years old. She has no idea whether it is male or female. Making conversation, she asks if it ever feels lonely, but it just looks at her blankly. What does “lonely” mean? She realizes that she can’t even remember how that feels. She invites it indoors for a snack, but having accidentally spilled coffee on it, takes it into the bathroom to undress it and run cold water over the burn. She then realizes that it is a boy, and also that any sense of shame or embarrassment appear to be entirely absent. She wants to test if this is really the case and, stroking his penis, asks if he wants to put it inside her. He is like the Adam of Eden she’d once read about in a picture book. But she can no longer feel any sexual urge—she knows what sex is, but its voice has left her. As they lie in bed together, they listen to the howls of her mother held captive next door, and laugh in exactly the same way with identical features: mother and child blend into one, and the innocence of Eden is destroyed.