[3] Teenager Karen Braden (Kelley Bohanon) is a troubled mental hospital outpatient who is taken by her father George and sister Isa to a government facility near the Craters of the Moon lava fields in Idaho.

[1] Principal shooting took place in Arco, Idaho, Craters of the Moon National Monument and Preserve, and Bruneau Sand Dunes State Park.

Translated, it means "Let it be perpetual" or "It is forever"; appropriate for a time travel film, it is also the motto of the state of Idaho.

[7] Fonda either neglected, or did not wish to renew his rights on this film, and according to several sources, the movie passed into the Public Domain.

Time described it as a "very deliberate and closely controlled film graced with a slow, severe beauty that makes its quiet edge of panic all the more chilling",[9] whereas Jay Robert Nash in The Motion Picture Guide declares it a "useless piece of drivel about an obnoxious group of teens".