Once and future collide when Ari, Merlin and the Knights of the Rainbow attempt to steal a magical chalice from Earth's medieval past in order to save humanity's future, irreparably entangling our space-age heroes with the original King Arthur.

Ari plays a risky game of lies and chivalry and Merlin confronts his nemesis, his older self, while all the time they must preserve the time continuum to eventually return to their own future. When the Lady of the Lake interferes, Merlin and Ari realise how much their future has been manipulated by her exquisite machinations of the past. Nin offers a way to release King Arthur's spirit from Ari's body - to end the cruel cycle that keeps them all prisoner - and allow them home. But at what cost?

This galaxy-altering conclusion unravels the dark truth of Merlin's origins and inspires a new hope for the Once & Future universe.