Books in the Museum Adventures series

Nick and Katie found a time machine in the basement of their grandfather's museum. Before they even know where they've traveled, Nick and Katie are chased through a castle, arrested for stealing, and thrown in a dungeon. Surrounded by darkness, rats, and filthy peasants, they hear talk of a name they thought was only found in storybooks … Robin Hood. Will Nick and Katie find a way back home, or will they remain prisoners to the infamous Sherriff of Nottingham?