Twelve-year-old AP is crazy about science but Kate, his older sister, has no time for him or his geeky experiments. All that changes when a curious abacus transports them back to Arthurian England. There, AP uses his knowledge to help them out of trouble. On a second adventure, they join the Sioux in Montana and witness Custer’s defeat at the Battle of the Little Bighorn. Then, in ancient Egypt, AP uses science again, allowing Kate—mistaken for a priestess—to perform ‘magic’ that amazes even the Pharaoh. Stalking them on every trip is a hooded stranger whose attacks show he’ll stop at nothing to snatch the abacus.

In-depth historical research helps set the scenes so well that readers will feel like they too have traveled back through time. And the science AP uses to help the two escape their time travel troubles is explained in easy-to-repeat experiments included at the end.