Travis, Paige and Bethany used the cylinder to time travel to Yuma, Arizona, where they figure out that a controlled catastrophe is about to happen in the current month of December and that it will have the effect of virtually destroying the known world. What that incipient disaster is has them running all over the continents, back and forth in time, and facing their nemesis -- a former humanitarian in cahoots with the sitting President and a host of other corrupt scientists and government officials.
Although some suspension of disbelief at the last minute saves and some of the science and politics is required, it is after all science/adventure FICTION, and should be read just for the pure escapist pleasure of it.
Seems that there is plenty of room for another book in this series and I will look forward to reading it as well.
Recommend -- enjoy!