Medieval Welsh literature is filled with references to non-human peoples who have visited Britain. There are the people of Annwfn, the 'Eingl' (angels), the Coranieid and the family of Llwyd Cil Coed. Some were friendly and some were hostile but all interacted with the Britons in history and legend.

At the same time there are certain reoccurring motifs which can be best explained by reference to modern science and technology. There are marvellous bottles which can keep milk fresh and liquid warm all day, hypnotic music which sends people to sleep, strange sounds of thunder which precede the arrival of non-human characters, and a singular floating glass fortress which forms the entrance to Annwfn, a subterranean world only spoken of in legend. This book critically examines medieval Welsh literature to answer the questions: Could some of our history and legends have been inspired by contact with alien races? And, if so, how probable is it?