If you were light years ahead in the future, perhaps your parents would let you spin the family travel vehicle for 20 minutes - it's just like riding a bike to them. But to use the time-travel facility on it is NOT allowed, not even to replace Gran's special sparkly glass artifact from the Elizabethan age (the 21st century!) So when Rosemary breaks the glass, and then breaks all the rules to get a new one, she has some surprises in store!