Here is a synopsis from "Publishers Weekly". You can also learn more about Zindel's novel at eNotes by following the link below.
"Jackson, a 15-year-old New Yorker, arrives at his anthropologist aunt's lodgings near Stonehenge, he gets a nasty glimpse of the murderous beast that the British Army has engaged his aunt to track down. As Jackson and his aunt, Dr. Cawley, determine, the monster, "Ramid," is an ancient mutant hominid that, in tune with the 19-year cycle of the moon, comes out of its home in abandoned flint mines to feed. When a bite from Ramid gradually puts Dr. Cawley in its thrall, it's up to Jackson and a comely local girl to hunt the monster. Zindel makes plentiful use of local scenery and Stonehenge lore and, for YA thrill-seekers, there's plenty of gore and creepy-crawly slimy stuff. The final showdown on the roof of Salisbury Cathedral is a spine-tingler."