A quiet September afternoon in Brecon, Wales and a Nazi SS officer mysteriously appears on the street. Entering a police station he shoots two police officers but later commits suicide. University lecturer Polly Probert picks blackberries along a disused Welsh railway track when a train passes and disappears. A lunch box thrown out bears a Reichbahn logo and a price in marks. When she and her colleague and lover Harry Long are working in a remote mountain valley they see a concentration camp and nuclear reactor. A Nazi officer arrives at what he expects to be a Waffen SS base but finds himself in a firefight with the British army. Polly discovers that every ten days she must shuttle between her own time and 1978 when Germany had won World War II. Anyone very close to her will go with her and come back. The Reich of 1978 is seen through the eyes of Ludwig Fürstner the cynical head of intelligence services. Adolf Hitler's son now rules and is planning a surprise nuclear attack on the USA. He intends to marry the head of concentration camps - and they hate each other. An American physicist explains multiverses as quantum wave forms and two waveform seem to be interfering. As interference increases one or maybe both universes must decohere, disappear. Polly's shuttling could be used to increase instability in the other-world and make it decohere to 1940. Polly meets her half sister (Janet) in 1978 who is also caught up in time shuttling. Visits become increasingly dangerous and Harry is badly injured in one transfer. Polly and Janet are on the run in snowbound Welsh mountains aided by Welsh partisans. As events unfold which universe will meet its Götterdämmerung?