Despite their vow to each other at the end of Once Every Never, best friends Clare and Allie once more find themselves in trouble—and travelling in time. Indulging in a low-key vacation at Glastonbury Tor, taking part in an archaeological dig while soaking up the sights of summertime England, the girls have promised each other: no time travel shenanigans; no involvement with dangerous Druids or villainous museum thieves; and definitely no weirdness about the fact that Clare is now seriously dating Milo, Allie's super-genius hottie cousin!

But when Allie makes an unexpected discovery at the dig site—a skull—the grisly artifact sends her spiralling back in time to a Roman encampment besieged by rampaging Celts. Caught between the Legions and the war band, Allie is rescued from certain death by Marcus, a young Roman soldier with a secret. As she struggles to survive in the past, Clare and Milo race desperately against time in the present to bring Allie home...before she loses her head OR her heart.