Deep within the gates of the fantastical, protected animal paradise known as Sanctuary City, stands Kerry, a young, rebellious ring tail possum. Kerry is obsessed with the world outside of the city walls and the adventures that lie there, amongst the animals of the Wild. In the centre of the utopian city, the Wishmas Day ceremony is taking place, when all the city's children pluck seeds from the sacred Wishmas Tree to make wishes of generosity and unity. In defiance of the city’s law, her loving father, and Petra, her pragmatic sister, Kerry does the unthinkable and sneaks into the Wishmas Tree at night to pluck the very last seed, intent on changing Sanctuary City, no matter the consequences. Kerry awakens the next morning, to a city blanketed in snow and ice, a WHITE Wishmas, but the initial novelty quickly gives way to crisis as the city's energy-generating waterfall freezes, feral mountain animals encroach on the city, and the sacred Wishmas Tree begins to die. The eccentric Keeper of the Tree, Yarra the Frilled-Necked Lizard, tells Kerry the only way to reverse the wish is by journeying through the forbidden Wild to find the fabled Sister Wishmas Tree, the only place where a wish to save the city can be made. They head off on their quest, with Petra in tow, and as the sisters navigate the perils of the Wild, they must also navigate and repair their broken relationship. On their quest to find the Sister Tree and save the city from an icy fate, the two discover the true meaning of Wishmas.