Saving the Unicorn's Horn

What can you do when your closest friend lives hundreds of years away?
Joe Hopkins is staying in York for October half-term. He hasn't seen Lucy since the beginning of September and he's really missing her. But when at last he slips through time again, he's alarmed to find himself in a different world altogether. This isn't Roman Britain. It's Jorvik, in the age of the Vikings.
Joe is relieved to discover that Lucy is here, but dismayed that his former enemy seems to be in this new world too. Can Joe protect Lucy from harm this time, and stop the prophecy of the unicorn's horn from coming true?