In 1907, Edward Runklemann knocks on the locked door of his house, only to find his wife living there with another man, who looks just like him. The two men scuffle and are arrested. A group of prominent businessmen, the Knights of the Carnelian, attempt to solve the mystery.
In 2010, Billy Boustany ditches work and visits and his old neighborhood in St. Louis. He stumbles into an alternate version of the city, which he calls “HD” (for “high definition”) in contrast to our world of “SD” (for “standard definition”). HD St. Louis is filled with strange buildings, bustling crowds, and raucous celebrations. Visiting this city becomes Billy’s secret hobby, the most fun he’s had in years.
When the stock market crashes in 1929, James Whittemore Hines, a brilliant, reckless stockbroker, loses everything. Overcome by shame, he leaps from the roof of his St. Louis office building towards the street far below. He wakes up in a hospital room. He’s in the same city, but no one knows who he is or anything about the stock market crash. Jim can’t believe his good luck and seeks new riches in this odd place. When a huge earthquake threatens the city’s future, he sets it on the path to becoming the teeming metropolis that Billy will encounter decades later. His greatest achievement is the Citizen Shareholder Plan, or the CSP, which gives every St. Louis resident a financial stake in the growth of the city.
In 1982, John Little, a TV repairman and Billy’s friend, meets Leonora Matsui, a woman from the HD city, on a distorted ham radio connection. They fall in love. Her mysterious uncle Martin intervenes to separate them, which leaves John heartbroken.
Billy’s daughter, Meredith, confronts him about his mysterious absences. He tells her his secret. She’s skeptical, but goes with him to HD. They explore together, discovering new wonders at every turn. He still avoids telling his wife, Carol, about his adventure. The longer he waits, the harder it will be.
By 1941, Jim Hines has become the powerful leader of the HD city. He discovers special pills that provide reliable travel between the two worlds. He steals products and technology from SD, then patents and sells them in HD. To protect his scheme, he transforms the Knights of the Carnelian from a social club into a secret organization to suppress knowledge of the SD world.
A letter summons Billy to a meeting in HD. It’s from the Knights. Their leader, Giles Monroe, warns him to stay away—or else. Against Billy’s wishes, Meredith enters HD on her own. She meets Diyami Red Hawk, a young Native American man. Billy crosses over to find her and they go with Diyami to the place where he grew up—Cahokia, a gleaming Native American city. Billy brings Diyami to visit SD, where the emptiness of our Cahokia depresses him profoundly. After Diyami returns home, Billy tells Meredith never to go back again. He fears the threats from the Knights.
Meredith ignores Billy’s warning and disappears into HD. Billy has to spill his secret to Carol, who thinks he’s crazy until she sees the other city for herself. They find Meredith, who has been kidnapped by the Knights, but fail to rescue her. Then the Knights kidnap Billy and Carol. Giles Monroe plans to keep them in his world forever.
In 1966, Jim Hines brings his protege, Martin Matsui, to SD for the first time. Jim is retiring and he wants to introduce Martin to his technology theft contact, a loud, coarse man who is celebrating the birth of his son. On the return trip to HD, Jim's heart fails.
After a few tense days, Martin Matsui, his niece Leonora, and John Little rescue Billy and Carol. They flee to Cahokia, where Meredith and Diyami are waiting. Martin can send them all home, but he warns them to never speak of HD to anyone. Diyami announces that he too wants to come to our world—to build a new Cahokia in SD.
After his return to SD, Billy Boustany is having trouble adjusting to normal life. He longs to visit HD again. He, Carol, and Meredith are helping Diyami begin his mission to create a Native American city at SD Cahokia.
In Washington, DC, a young intelligence analyst, Lisa McDaniel, notices strange radiation events in St. Louis that were observed by a secret spy satellite the previous year. She investigates until her work is stopped by Ralph Pellegrini, a deputy director of the CIA.
Diyami and Meredith travel to Native American communities to promote his vision for Cahokia, but no one is interested. Diyami is profoundly depressed. He takes a solo canoe trip on the Mississippi to seek a vision to guide him. He encounters a bizarre figure on a remote island.
Lisa makes an unauthorized trip to St. Louis—she’s obsessed by the mystery she discovered. Billy and Carol find her snooping around their building. They confront her and she partially reveals what she’s doing. Can they trust her with the secret of HD?
John Little is living with Leonora Matsui in HD. He explores the intricacies of HD technology and helps Leonora search for a solution to problems affecting Hydraulic Brick’s electricity-generating bricks, which are the primary source of power in HD. They need to build a computer for data modeling, but computers don’t exist in HD. Uncle Martin gives her some of the top secret pills. Leonora comes to SD to see Billy and Carol and get computer parts. They agree to help if she shares the pills so they can visit HD again.
Giles Monroe is embarrassed by bad publicity and the failures of the technology theft scheme, which no longer produces the lucrative profits he wants. He has a new plan.
Diyami’s mission in SD begins to gather steam, with the help of Stan Huffman, a successful Native American lawyer.
Billy and Carol travel to HD, where they notice a disturbing change—people are afraid of the police. Surveillance cameras are going up on street corners. John realizes that, despite his homemade computer, he doesn’t have the math skills to solve Leonora’s brick problem. Billy and Carol have an idea—Lisa McDaniel could help. Billy recruits her to come to HD. She is astounded when she sees this new world. Lisa and John solve Leonora’s problem and save Hydraulic’s electric brick business.
Lisa suggests a new project, mapping the electromagnetic barrier between the worlds to look for a way to travel without the Knights’ pills. They succeed and discover a weak spot in the barrier, a “bubble” over Cahokia and St. Louis, which is what allows travel between the worlds.
They celebrate by going to the biggest festival in HD St. Louis, where Lisa sees a shocking sight. Giles Monroe is riding on a parade float with Ralph Pellegrini next to him. What is Pellegrini doing here?
Billy discovers that Pellegrini and a CIA team are behind the surveillance camera system. He, Carol, and Meredith are its prime targets. Lisa and John propose a plan to thwart the CIA threat—“pop” the bubble and end all travel between the two worlds. Before they do, Leonora gives Diyami plans for building Hydraulic’s electricity-generating bricks. It’s payback for the decades that the Knights’ stole technology from SD.
John and Lisa pop the bubble, which unleashes a deafening boom across both HD and SD St. Louis. Six months later, Diyami announces his new company in SD to build electricity-generating bricks. Diyami and Meredith follow a prophecy and discover a second bubble at the ancient Native American site of Poverty Point, in Louisiana. They keep it a secret.