Cora hasn't spoken to her best friend, Quinn, in a year. Despite living next door to each other, they exist in separate worlds of grief. Cora is still grappling with the death of her beloved sister in a school shooting, and Quinn is carrying the guilt of what her brother did. On the day of Cora's twelfth birthday, Quinn leaves a box on her doorstep with a note. She has decided that the only way to fix things is to go back in time to the moment before her brother changed all their lives forever--and stop him. In spite of herself, Cora wants to believe. And so the two former friends begin working together to open a wormhole in the fabric of the universe. But as they attempt to unravel the mysteries of time travel to save their siblings, they learn that the magic of their friendship may actually be the key to saving themselves.