Found by Margaret Peterson Haddix is Book 1 of The Missing series, and not only does it have plenty of plot, but it gives tantalizing hints at the many plot directions the rest of the series can take. Before we go any further, I'll definitely have to issue spoiler alerts.
Jonah, thirteen, is adopted and has always known it. His younger sister Katherine is not. Though his parents' constant reading of "how to help your adopted child adjust" books and hyper-sensitivity to how he might feel make his eyes roll, theirs is a normal family. Then an unusual thing happens: Jonah makes a new friend, Chip, who also happens to be adopted. But Chip's uninvolved parents never told him. Chip doesn't have a clue until the weird stuff starts happening.
They both get strange anonymous notes: "You are one of the missing," says the first. The second reads, "Beware, they're coming back to get you." The investigation by Jonah, Chip and Katherine is kicked off by Chip's anger at his parents' deception as well as the notes themselves. Breaking into a safe at Chip's house, the kids find among the official records a jotted name and phone number. Calling it, they reach an FBI agent.
Things get stranger when the FBI agent agrees to meet with Jonah's family but deliberately antagonizes them hoping they will stop asking questions. Katherine, uncharacteristically spooked, insists she saw a man appear and disappear into thin air. Jonah doesn't believe her but can't deny that there's a file on the agent's desk that wasn't there before. He creates a distraction while Katherine snaps pictures of the papers it contains with her cell phone.
Now they have lists of people and phone numbers, some of whom are termed "missing" and the others "survivors." Thirteen years ago, they learn, a myterious plane bearing the insignia "Tachyon Travel" landed unauthorized at an airport -- carrying thirty-six infants and not a soul else. One of the airline clerks who worked that night agrees to meet with them and shares the belief she has arrived at: because tachyon means faster than the speed of light, the infants were probably adults when they boarded the plane, and they arrived in the present as infants to be adopted by 21st-century families. The flight crew, able to step into time warps, simply disappeared. In fact, the thirty-six infants were gathered by time-travelers from prominent but unfortunate lives in the distant past, purportedly to be given a chance at a better life -- or life at all -- with adoptive American families. One among their number, they are told, is Virginia Dare, lost with the rest of the Roanoke Colony. Two are Princes Edward and Richard, vanished from the Tower of London in the 1400s. Others are Anastasia Romanov and Charles Lindbergh III.
Found ends with Jonah, Chip and Katherine on their way to the 15th century, so it definitely doesn't stand alone. With potentially thirty-some countries and historical periods to visit, Haddix has rich setting and plot possibilities to draw on. Her portrayal of the sibling relationship between Jonah and Katherine and the boys' respective feelings about adoption are well handled. It will be interesting to see how Haddix makes use of her material in books to come. And I for one really want to know which unfortunate famous historical person Jonah is.