The writer C.S. Lewis is invited to visit a lab in 1940’s Cambridge England where he is shown a device called a "chronoscope" that does to time what a telescope does to space. The chronoscope shows strange images of a dark tower ruled by men with stingers on their foreheads. An accident with the machine results in minds being switched between Cambridge and the dark tower. In the present, a precocious teenager and her mysterious nanny explore and ultimately complete this work of fiction that was discovered after C.S. Lewis’s death.