Set in the summer after Gavin's rescue, Meg & Sue are visiting Gran in Wales. Gavin also turns up - hostile, surly and totally unlike the one they rescued. He takes off at dawn into the surrounding hills, coming back way after dark. They decide to follow him and soon all three are drawn into unknown places - and times! These turn on a legend that King Arthur, with Sir Gawain, stayed at a nearby castle during his reign, setting off a series of catastrophic events in which now the three become entangled and in peril of their lives. (During their rambles that castle comes and goes, changing the terrain around it.).

Close by Gran's cottage is an ancient great hall. Pre-dating medieval times it is in perfect condition and tastefully modified for use by the mysterious (absent) owner. When they ask Gran for the meaning of the hall's name she tells them that it means cataclysm.

Ryferthwy Hall

Hall as seen at one stage in the book

Shifts in time and space escalate at an alarming rate. As they race for the hall, they find it as the inn it was in medieval times - and a bunch of riders setting out into the night.