The time-tripping Faradays

When twenty-fifth century time-travelers Dawkins and Hypatia find a plastic artifact among the Neanderthals, it is an anomaly--but on their next assignment to Japan in 1595 they find much more significant evidence of tampering, using virtual reality to induce belief in a demon tengu, and causing mass hysteria.

Their latest adventure takes teenagers Dawkins and Hypatia back to thirteenth-century Nottingham and Sherwood Forest, but when they find a time-traveling device in a cave they find themselves far in the future--and closer to the forces that are tampering with time.