The Golden Crucifix

On a sultry, threatening summer afternoon, Siobhan, Penelope, Alyce and Emily are picnicking.
When the storm breaks they run for shelter - straight into an unfamiliar world which they don't understand and where they seem to be in some danger.
They have somehow (but how?) arrived, as they gradually come to realise, in some dark corner of the past, and the people they have to deal with are plotting a terrible massacre. Where and of whom, the girls cannot at first discover.
Siobhan, the eldest, is also the most concerned, not only because of the real danger to herself and her sisters, but also because she feels she must warn the authorities in the person of the King's Secretary, Robert Cecil - and yet she is fascinated by Catesby, the central conspirator.
Because she has taken little interest in history, she does not know how the story will end...