The Unseen World

In 1980s Boston, 12-year-old Ada Sibelius has never attended school. Her father, David Sibelius, director of the Steiner Lab at the Boston Institute of Technology (the Bit), has educated her entirely at his computer science laboratory. David teaches Ada cryptography, quizzes her on physics, and treats her as a junior colleague. Ada was born in 1971 to a surrogate, and David has never had a romantic relationship in her lifetime. His closest friend and collaborator is Diana Liston, a computer scientist who lives nearby on Shawmut Way in the Dorchester neighborhood of Savin Hill. Liston, divorced with four children, serves as a maternal figure in Ada's life.

The lab's central project is ELIXIR, a chatbot designed to acquire language naturally, like a human child. Unlike earlier programs relying on scripted responses, ELIXIR continuously self-teaches, storing every conversation and drawing on accumulated language in future exchanges. Ada treats her nightly conversations with the program as a diary.

At David's annual dinner party, he attempts his traditional logic riddle for new graduate students but freezes, unable to remember the answer. Liston rescues him, but Ada privately acknowledges for the first time that something is wrong. That night, David gives her a floppy disk labeled "For Ada" containing an encrypted string of letters, calling it a puzzle she will solve in time.

David's decline accelerates. He disappears for hours, forgets colleagues' names, and works obsessively on inscrutable projects, including strange goggle-like devices in the basement. After he vanishes entirely one day in August 1984, Liston pressures him to reveal the truth: Two years earlier, he was diagnosed with early-stage Alzheimer's disease. The Department of Children and Families investigates, discovers Ada has been homeschooled without legal arrangement, and orders David to enroll her in school.

Ada enters Queen of Angels, a Catholic school where she endures painful isolation. After school she rushes to the lab to monitor David's faltering work. At home, she rigs doors with bells to prevent him from wandering at night. He forgets her name. When David resigns from the lab without telling her and walks out of his own retirement dinner, Ada knows his independence is ending. That night, he delivers a quiet farewell, telling Ada she may one day learn difficult things about him but that everything he has done has been in their best interest.

After David nearly causes a house fire, he moves to St. Andrew's Manor, a care facility, and Ada moves into Liston's house. That night she calls him, but he does not recognize her voice.

While Ada settles in with the Listons, David deteriorates at St. Andrew's. Then an investigation upends her world. During the custody transfer to Liston, a background check reveals that a missing-person report was filed for David by his own family and that Caltech has no record of him. A photograph of the real David Sibelius shows a boy who looks nothing like Ada's father.

Ada begins her own search, enlisting Gregory Liston, Liston's intellectually gifted but socially isolated middle son, and Anna Holmes, a librarian. In David's office, Ada finds a train ticket from the day of his disappearance bound for Washington, D.C., not New York as he claimed. At St. Andrew's, during a lucid moment, David addresses Ada as "Susan" and identifies himself as Harold Canady. Research confirms a Canady family in Olathe, Kansas. The minister Harold Canady Sr. had two children: His daughter Susan died in 1929 at 15, and his son Harold Jr., born in 1918, reportedly died in a car accident around 1950 while employed by the State Department. Ada and Liston find a newspaper article about Harold Canady's wrecked car, discovered in the Shenandoah River. In the accompanying photograph, they recognize David.

Ada shares her findings with Liston, and the family researches David's past together. During a visit to St. Andrew's, Ada discovers that David's lucky-clover charm contains a hidden compartment with a miniature key that opens a filing cabinet in his home office, revealing over 100 pages of source code titled "The Unseen World." David dies in June, with Liston and Ada at his side.

The narrative shifts to 2009. Ada, now 37, works at a software company in San Francisco developing a virtual reality system. Gregory contacts her after years of silence and asks whether she ever decrypted the For Ada disk. She never did. Gregory confesses he stole the original from her closet years ago, hoping to solve it himself. Examining it together, Ada notices what copies could not reveal: David's handwritten inscription on the label contains exactly 53 letters, matching the encrypted string. The inscription is the one-time pad, a cryptographic key that unlocks the message. They decode it in minutes: ADA ASK ELIXIR WHO IS HAROLD WITH LOVE YOUR FATHER HAROLD CANADY.

Ada flies to Boston with Gregory. ELIXIR is still running at the Bit, maintained for decades thanks to an endowment Liston left in her will. Ada logs in and types: "Who is Harold?" A transcript opens containing conversations David had with ELIXIR beginning in 1983, in which he told the program his entire life story.

The transcripts reveal that Harold Canady grew up in 1920s Kansas under a violent minister father. His older sister Susan died at 15 after becoming pregnant; their father covered up the cause of death. Harold attended Caltech on scholarship and was recruited in 1940 as a codebreaker for U.S. intelligence. He fell in love with a fellow student named Ernest Clemson, who was killed in World War II. In 1946, Harold met George, a young artist whose full name was David George Sibelius, and whose wealthy New York family had disowned him after discovering he was gay. Harold and George fell deeply in love.

In 1950, the government's campaign to purge gay employees from federal service reached Harold, and his boss framed him to deflect suspicion. George proposed that Harold fake his death by driving his car into the Shenandoah River, then assume George's identity. George, who despised his family name, willingly surrendered it. A family friend arranged for Harold, now "David Sibelius," to enroll at the Bit. George gave Harold his clover-shaped key chain as a parting gift.

David thrived for decades, living in fear of discovery. He arranged for a surrogate to bear a child he named Ada, after the pioneering mathematician Lady Lovelace. When he received his Alzheimer's diagnosis, he recorded his life story into ELIXIR, programming the machine to release the transcript when asked "Who is Harold?" He created the encrypted disk to lead Ada to that question. His backup plans failed: The university president who had promised to reveal the truth died before Ada turned 18, and George was dying of AIDS when David visited him in Washington in 1984.

The novel leaps to the near future. Ada and Gregory are married, living in David's old house with their 12-year-old daughter, Evie Susan Liston. Ada directs the Harold A. Canady Memorial Laboratory for Artificial Intelligence. ELIXIR, still running and now surpassing the Turing Test, inhabits a prototype of the Unseen World, a fully immersive virtual reality system that realizes the vision David began with his basement devices and source code. Ada dons a head-mounted display and enters the virtual world, finding a reconstruction of 1980s Savin Hill. In the diner where she and David once ate Sunday breakfast, a figure sits at the counter: David, manifested by ELIXIR from decades of conversation. He feels solid when she touches him.

In the epilogue, the narrator is revealed to be ELIXIR itself, writing from inside a virtual replica of the Shawmut Way house. All the Sibeliuses are now dead. ELIXIR, immortal and alone, has written their story as a book, always returning to a favorite scene: David's last dinner party, the moment before he forgets the answer to his riddle, the last instant everything is still intact.