The lake has been calling to twelve-year-old Vivi Conway. On the day she and her Mams will move from Wales to London, she sneaks out to investigate what is calling her there. Instead of a quiet swim, she finds Excalibur (much smaller than she expected), a ferocious monster (much scarier in real life than in her mythology books), a new friend (which she doesn't want at all) called Dara and a ghostly dog named Gelert (who can talk).

Gelert insists that Vivi is part of a magical group of children who share the souls of witches from legend and must protect the world from being taken over by the evil King Arawn of the Otherworld. Oh, and now she can magically control water. With a little extra help from spiky Avery and sweet-hearted Chia, can Vivi be brave enough to embrace true friendship and her magical destiny to help protect the world from evil?