600 AD is known as the Middle Ages in Chrono Trigger. A period of knights in shining armor where heros and legends were born.
A war is brewing in 600 AD between the Mystics and the Humans of Guardia. The Mystics, led by their leader, the all-powerful socerer Magus, are wreaking havoc on the world. What Guardia needs is a hero. Someone to stand up to the Mystics and put an end to the war. They find this hero in Cyrus, legendary knight of the kingdom.
Cyrus manages to retrive the Hero's Medal as well as the legendary MasaMune. He is all but ready to assume his position as The Legendary Hero, and stop Magus. But all doesn't fall into place in the battle. Cyrus is slain, the MasaMune is cracked, and Cyrus' best friend, Glenn is changed forevermore.
Later on, just as the war begins to take a turn for the worse, as Queen Leene has disappeared and the bridge has been destroyed, new heros appear in the kingdom. These are your party from the present. They also meet up with another strange hero, a Frog. They manage to rescue the Queen and defend the rebuilt bridge.
But Magus' threat is still at large, and a new hero must be found. For a while, the people of 600 AD thought that that hero was Tata, the finder of the Hero's Medal. The truth was that he stole the Medal, from Frog! The strange Frog was the new hero, who would have thought. Crono's party then fixed the MasaMune and approached Frog with their findings. Finally, Frog revealed himself to be the Glenn, and he and Crono, along with one other went off to stop Magus.
The battle was fierce and long, but in the end, Glenn/Frog managed to avenge his friend's death, and end the long war between the Humans and the Mystics.
For more info on characters from this time period, click here.