Wikipedia- An elliptical region, including the island of Nantucket, Massachusetts and the United States Coast Guard ship Eagle, is transported by an unknown phenomenon (called "The Event") back in time to the Bronze Age circa 1250s BC (corresponding to the late Heroic Age of the Trojan War).
As the truth of what has happened sinks in panic grips the island. Sherrif Jared Cofflin is given emergency powers and begins organizing the people to help produce food for the island so they can feed themselves. Meanwhile Captain Marian Alston takes the Eagle to Britain, with Ian Arnstein and Doreen Rosenthal as interpreters, where they trade Nantucket made goods with the "Sun People", a tribe that has been steadily invading the island, for grain. As a gift the Sun People chief gives Marian a slave, Swindapa, a captured female "Earth People" warrior. Swindapa is freed and decides to stay with Marian. The Eagle leaves for Nantucket but takes with them Isketerol, a Tartessos merchant who hopes to learn from the Americans.
While the people of Nantucket work for their survival, William Walker, a lieutenant on the Eagle, decides that with modern technology he could become a king in this time. With the help of Isketerol and others, Walker convinces some naive environmentalists to steal a ship and kidnap Cofflin's wife so they can give guns to Native Americans. Meanwhile Walker and Isketerol steal another ship and return to Britain to recruit soldiers for their eventual takeover of Greece. Marian decides to rescue Cofflin's wife first and saves her after defeating an Olmec army.
Time passes as Walker solidifies his control over the Sun People and Nantucket creates a new government and prepares to take down Walker. Marian returns to Britain with a small army and uses Swindapa, who has become her lover, to convince the Moon People to fight with them to defeat Walker. Both sides meet at the Battle of the Downs and though Nantucket and its allies are victorious, Walker manages to escape with his followers to Greece.