Original: The plot of the trilogy consists of the Guardians of Time (the Guard), trying to protect the past, present, and future by traveling into the past to thwart their enemies, the Order of Chaos (the Order), who are trying to change past events to give themselves more power.
Simplified: The trilogy is about a group of people called the Guardians of Time (the Guard), going into history to save the world from the Order of Chaos (the Order), who are trying to history to give themselves more power.