This book was illustrated by at least by Piers Sanford (UK) and James Watling (US). HarperCollins recommends its 2004 edition for ages 8 to 12.
Omri and his father learn that Little Bear needs their help as American colonists are starting to head into his people's land. The two realize that they must travel back in time and lend their aid but need to figure out how to do it. They have the key but they need something big enough to hold them. Omri's chest that he and Patrick used before was destroyed in the tornado they accidentally brought back and it isn't big enough to hold them. Omri's dad decides to use the car as the time machine and comes up with an elaborate plan to pretend to go on a camping trip as a cover for their time travel, but they realize that the key won't work in the car and start to despair. Omri tells his father everything about what he learned and shows him the Account.
After reading that, Omri's dad figures out that Jessica Charlotte could possibly create them a magical copy of the car key so they can travel back in time. The two also figure out that Omri has inherited some of her psychic power and that's most likely why he can tell when one of his friends from the past needs his help. It gives Omri the power to sense various things, like being able to figure out what's going on with Jessica in her time even though she was so weak at this point when she wrote the Account that only a few words can be made out: she's learned that her brother-in law has died and is going to try to drown herself. The two feel that they must bring her forward no matter what. Omri later brings her, initially planning on waiting, but his psychic gift senses her in trouble and causes him to bring her forward. She's drowned herself and isn't breathing so Omri summons Matron, who manages to revive her. He tries to comfort her about what happened and when she hears that he's Lottie's grandson she agrees to make a magical copy of the car key. He brings his father and they give her the key and send her back. That night they summon Little Bear to get something of his to use to travel back, and he gives them his wampum belt. He also agrees to have Bright Stars make toys for them to travel into when they journey to the past.
The next day Omri's father summons Jessica Charlotte without him and she returns the original car key and gives them the copy, but unfortunately the scale difference means that the key is too small to see, and hence is useless to them. They decide to go camping anyway and bring Gillon along, but Omri has an ivory elephant from India and Gillon is leaning against a knapsack from that time also and they suddenly find themselves in India about 90 years before. Omri figures out that Jessica Charlote's attempt to create the magical copy of the car key where she bent her entire will on it probably caused the original car key to become magical. He figures his father will realize what happened and bring them back by shutting off the car. During the trip Omri and Gillon are marionettes — Gillon, embarrassingly enough, is a female figure — and a couple of children spot them moving on their own and try to show them off. Feeling bad for the children Omri goes along with it but when the children's mother shows up both Omri and Gillon are dropped and Gillon hits his head. Thankfully before they can get hurt further they're pulled back to their time where Omri finds they never left the house as the father got pulled back to Little Bear's time due to the wampum belt being in his pocket and were pulled back when Omri's mother found them and shut off the car fearing they were dead. Omri's father ended up in a faceless doll but Little Bear promised that next time the dolls would have faces.
The two decide to pretend to go camping but head to the top of a nearby hill and time travel from there. They summon Patrick to send them there and back but Patrick is unhappy he won't be going until he realizes that he has the cupboard and can call any of their friends back. He sends them back and they meet up with Little Bear. The two eventually advise Little Bear to take his tribe to Canada but are attacked by two settlers. They set fire to the longhouse and start killing Indians. Old Clan Woman, the oldest and wisest member of the tribe manages to scare them off with Omri and his father's help, but she is killed by the two retreating men. Little Bear and his tribe escape and start the trek to Canada but Omri and his father are forgotten in the rush. They escape outside but are trapped in the stockade and the two men return and go to kill them. Before they can succeed Little Bear returns, having remembered them after hearing of their heroic actions earlier, and kills the two men and rescues them. He also retrieves a false face mask from the longhouse. He apologizes to them for leaving them behind and is ashamed as Omri is his blood brother. He tells Omri he is no longer a boy: Omri is now a warrior. He leaves the two a moccasin to use as a shelter until Patrick brings them back in the morning, and leaves after saying a final goodbye.
The next morning they are returned to their time by a frantic Patrick who explains that he brought Boone and Ruby Lou but lost them. They eventually find them in a septic tank and rescue and clean them up. Patrick also reveals that he planned to bring Little Bear to the present while they were away but Sergeant Fickets convinced him not to. A couple of weeks later Omri's mother brings him the wampum belt and he realizes that she knows the truth. She reveals that she, like Omri, has inherited some of Jessica Charlotte's psychic powers and, seeming to read his mind, explains that she's known the truth since the beginning but has kept it to herself. Omri's brothers have no idea and never will. They both agree that it's time to end the adventures for good. Omri closes his figures and the magical car key in the cashbox and decides to give the magic key back to his mother so he will no longer have the temptation to use it to bring the others back again. He first sends the wampum belt back and senses through his psychic powers that Little Bear and his tribe have successfully and safely reached Canada and reveals this to his father along with the fact that his mother knows the truth. However an apparently psychic dream indicates that perhaps he will still have further adventures.