So begins the diary written by Catherine Morgan as she describes the events of a Monday in 1810 Regency England. When another Catherine Morgan, who lives in present-day New York City, receives the diary, she is drawn into the mystery as the present Monday begins to resemble the Monday in 1810.

Writing in the diary, Catherine pleads for help by hinting this particular Monday can be changed as long as the day doesn't end the same way. Catherine and her brother XJ desperately try to unravel the clues from 1810, but this is complicated since the chapters of the diary become visible only when the same events have taken place in the present.

When a betrayal endangers the lives of those in 1810, present-day Catherine and XJ have less than 24 hours to uncover the mystery before the past repeats. Unfortunately, they think someone close to them might be conspiring to keep the mystery unsolved. If their Monday ends the same way as the diary of 1810, the events of the past will be forever repeated.