Quest for the Future

Regarding film technology, one thing cannot happen. The image cannot change. Once exposed, it's burned into the celluloid or similar material for the life of the changed material until it decomposes. Reversal or alteration of any kind is not possible.

An image changed.

The discoverer of the film can searches for the distributor from which the film was rented. That discovery leads him into the future. Expanded from an earlier story, this novel-length version appeared first in 1970.