Plotinus's system begins from the One, a reality so radically transcendent it is beyond being, beyond thought, and beyond any predicate that could be applied to ordinary things, which nonetheless generates the rest of reality not by choosing to create it, an act that would imply the One lacks something it needs to obtain by acting, but simply by being what it is, overflowing of its own perfect fullness the way, in Plotinus's own recurring image, the sun radiates light without diminishing itself or making any decision to do so.
From the One emanates Nous, divine Intellect, containing the Platonic Forms in eternal contemplation of the One; from Nous emanates the World Soul, which in turn generates the material world at the furthest possible remove from the original unity, matter itself standing at the very edge of reality, nearly non-being, precisely because it is maximally distant from and least unified with the One that all reality ultimately flows from. Crucially, this entire process is necessary rather than freely chosen: on Plotinus's account, the One could not fail to emanate any more than the sun could fail to shine, which is exactly the feature of the doctrine that later monotheistic critics, committed to a God who freely chooses to create, found hardest to accept, since it appears to make the world's existence a matter of the divine source's nature rather than of genuine divine will or decision.