Voyage with the Vikings Imagination Station

Mr. Whittaker, a kind but mysterious inventor, has sent cousins Patrick and Beth to Greenland through the Imagination Station. It's the year 1000, and they meet Viking Erik the Red, who is angry about the new God. Mr. Whittaker wants the cousins to find a sunstone, but what does one look like? And what does it have to do with the mysterious letter found in the inventor's workshop? Most important, can Patrick and Beth find a Sunstone before Erik's son Leif sails away with the Imagination Station on board his ship?