What if one summer could change your life? For Frankie and her five best friends growing up on the glorious beaches of Cornwall's St Felix, life couldn't get any better. Late one night, the friends discover an old box washed up on the shores and they learn the tale of the St Felix mermaid who grants wishes to those who find her treasure. Even though they know it's just Cornish folkore, the friends decide to make a wish for what they each want out of life - most importantly, they want to always be together . . .Over the course of the next few decades, as weddings, funerals, birthdays and more bring the friends back together, lost loves, broken promises and buried secrets just as quickly tear them apart. But when they reunite one last time in St Felix, will it be for good? And will their wishes finally be revealed?