Half-Life takes place in an alternate present different from our own only in that radioactivity in the air and water and food has created an ever-growing population of twofers, who are now the fastest-growing minority in the country. ([Nora] and [Blanche Olney]'s mother conceived the twins in a late-night romp on a bus with a geologist from Nevada whose father died from fallout.) In the pro-Togetherist culture of San Francisco, Nora and Blanche share a house with Audrey, a filmmaker and twofer wannabe, and Trey, a somewhat shady character with a host of illegal yearnings, who, like Audrey and Nora, does phone sex for a living.
All would be well in this kitschy, politically correct, sexually open community except that Nora has a pulsing desire that cannot be satisfied: She aches to be a "singleton" - solo, on her own, alone. She has felt alone for more than 15 years, since Blanche went to sleep and never woke up. So if Blanche isn't really there except as an impediment to solitude, why not dispose of Blanche altogether? Nora doesn't want to be a twofer, loathes the cult of Togetherism, keeps a journal of twofers memorabilia - The Siamese Twin Reference Manual, which she deems a "devotional to self-loathing."
Long before Nora heads for London to pursue the single life, Blanche seems to be rearing her sleeping, vestigial head: Her eye opens in Audrey's film of the twins, a woman the lesbian Nora is not attracted to accuses her of molestation, mysterious messages appear on Trey's voice recognition software, a plate of cupcakes goes whizzing through the air at a party, Nora awakens in places she does not remember having gone and other unsavory moments. Nora blames Blanche, Audrey tells Nora she needs to embrace her other self, Nora's mother suggests Siamysticism.