Anne and Julie, on a school field trip to the National Aquarium in Baltimore, are having fun looking at the aquatic creatures and talking about other students. But suddenly, in one darkened deep sea room, Anne vanishes and a strange girl in ragged clothes appears in her place. Anne has switched places in time with a runaway slave named Millie. Now in the year 1849, Anne is captured by a cruel slave catcher and held as a thief on his sailing ship. Scared but determined, Julie must figure out how to rescue Anne. But if she does, will it mean sending Millie back in time to the slave catcher?