Jenna and Caleb go with their mother to visit a great aunt whose attic she is helping to clean. Their Uncle Al is out ranging, but the kids are allowed to look in his office, but not touch his desk. Of course they do, and they find instructions on how to time travel. This is too good to leave, so Jenna takes the book with her. The next day when they go back to the house, the children, along with their always prepared friend Ari, go to the crest of the hill described as having a "reverberator", deliver the chant as instructed, and travel back in time! Immediately, they are attacked by a short-faced bear, but Jenna chants another incantation and a mythic bird comes to their rescue and flies them off. The three see an amazing array of creatures and natural formations, which luckily Ari knows all about. There is plenty of danger, and the kids have to find someway to get back home... and when they do, their Uncle Al catches them and reprimands them for sneaking into his things and undertaking a dangerous adventure. But he is secretly pleased, and more adventures may be in the works for the trio.
I do love time travel, and this is rich with details about a specific point in the past. The graphic novel format might draw in readers who normally stay away from time travel books.