Evie is tired of being left out by her brother just because she's a girl. So when a unicorn appears in her backyard and tells her she is the fearsome warrior destined to save Rainbow Valley, what can she do but agree to help?But it's going to be harder than she thought. The sprite Ruby Evergreen says only maths and magic will save the magical world and Evie hates maths. Can Evie discover how to defeat the Goblin King and still get back home in time for bed?