The Spartan Sacrifice: A Jump in Time Novel, Book Four

With Victor Stahl's plot to rule the world on an unstoppable path in the present, seventeen-year-old time jumper Dan Renfrew and his partner Sam realize that their only hope of putting an end to Victor's scheme is to jump into the past again. They arrive in Greece in the year 480 BCE just before the legendary Battle of Thermopylae, where a heavily outnumbered force of Spartan warriors and their Greek allies bravely try to prevent Greece from being overrun by the invading Persian army. But when a horrible accident happens in the past, Dan and Sam discover that Victor's plot is the least of their worries.