Before launching into mystical exegesis, a brief word of caution: mystic that he is, Plotinus is far, far better read for oneself than explained by someone else—but at times he can be so obscure that some explanation is necessary to provide a point of entry. In what follows I'll try to lay down such a foundation by defining the terms of his metaphysics—as far as it is possible to define Plotinus' terms—and by explaining the various ways in which they fit together and interact.
There are three fundamental principles in the metaphysics of Plotinus; they are the One, the Divine Mind (or Intellectual-Principle), and the Soul. They are not hypothetical explanatory principles, but instead define reality itself through their interactions; the existence of each is predicated upon the existence of the others, and they are arranged in a sort of hierarchy of contingency and necessity (each is necessarily contingent upon its others) with the One at the topmost point, and Divine Mind and Soul arrayed beneath it in that order. But the relationship between the One (at the top) and Soul (at the bottom) is not strictly always one that is mediated by the Divine Mind between them; they also commune directly with each other in a sense, even as they interact indirectly through Divine Mind.
The One is arguably the most important element of Plotinus' philosophy. (He claims that it is not his invention, and rather that it was developed by Plato—viz. the form of the Good—and perhaps has an even longer history in the work of the pre-Socratic philosophers.) Unsurprisingly (by virtue of its name), it is the overarching unity that is common to, and is the source of, the entire universe. (Plotinus also sometimes refers to the One as God, which puts into context what he is getting at with its meaning: it is not just unity, but also could be thought of as Creative Reason itself, or as the Good itself, etc.) Precisely because it is unity itself—simplicity itself—its nature is fundamentally indescribable: everything else hinges upon it to such an extent that it is no longer an object of concern at all. Since it informs the entire sensible and intelligible universe it is not meaningful to talk or even think about it, because our entire basis for speech and thought are contingent upon it. (That is, the One is not necessarily incoherent; what is incoherent is any attempt to describe it.) Thus it is necessary that the One be at the topmost point of any divine hierarchy.
The two principles that can be derived from the One, then, are Divine Mind (or Intellectual-Principle) and Soul. Divine Mind is derived first because of the way in which it can mediate between the One and the Soul, and because according to Plotinus' philosophy, intellectual contemplation of the One is the greatest kind of life—so that if Soul is the sine qua non of life, Divine Mind is its principle. Moreover, because of this privileged place in relation to the One, Divine Mind is naturally the closest to it:
the Soul, for example, being an utterance and act of the Intellectual-Principle as that is an utterance and act of The One. But in soul the utterance is obscured, for soul is an image and must look to its own original: that Principle, on the contrary, looks to the First without mediation—thus becoming what it is—and has that vision not as from a distance but as the immediate next with nothing intervening, close to the One as Soul to it. (V, 1, 6)
That is, Divine Mind can look directly to the One without needing an intermediate step in between; and Soul bears the same relation to Divine Mind as Divine Mind does to the One, standing so close as to not require mediation between it and its object. In this way, Divine Mind necessarily mediates between Soul and the One—Soul is affected by the One through the filter of the Divine Mind, as its "utterance and act".
Despite all things being always already predicated upon the One, they can still be responsible for particularities. For instance, for Plotinus, Soul is the eternal source of all life (though not its principle, because its principle is Divine Mind):
Let every soul recall, then, at the outset the truth that soul is the author of all living things, that it has breathed the life into them all, whatever is nourished by earth and sea, all the creatures of the air, the divine stars in the sky; it is the maker of the sun; itself formed and ordered this vast heaven and conducts all that rhythmic motion: and it is a principle distinct from all these two which it gives law and movement and life, and it must of necessity be more honourable than they, for they gather or dissolve as soul brings them life or abandons them, but soul, since it never can abandon itself, is of eternal being. (V, 1, 2)
Soul, then, governs over nature and in turn is governed by Divine Mind, which itself is guided by contemplation of the One. (It is 'governed' by Divine Mind insofar as it is "the life-principle carrying forward the Ideas in the Divine Mind" (V, 1, 7).) Soul in turn guides body or matter, which it shapes—these are the things that comprise nature. Because Soul is embodied in so many different things, it is a multiplicity—the continuity of its nature is unifying, but it is a multiplicity within unity, and a unity in multiplicity. (This lesser grade of unity comes about because Soul is not the source of its own unity—it derives it from the One. Only the One can be truly unified and self-complete, because it is the source of its own unity.) This aspect of its nature indicates that it is further away from the One than the Divine Mind—not only since it requires Divine Mind to mediate between it and the One, but because of the multiplicity of its nature. Clearly anything that is not itself a unity has fallen away from the perfect and self-contained wholeness of the One; Soul is such a fallen principle, poor in unity compared to the Divine Mind and the One that rank above it in the hierarchy described above. But this is not to diminish the importance of soul: because it is shaped by Divine Mind and because it shapes the whole of nature, because all individuals participate in its unity-in-multiplicity, it is just as necessary as the other two. In this way it also serves to define them, by negation: for example, "[w]hat the Intellectual-Principle must be is carried in the single word that Soul, itself so great, is still inferior" (V,1, 3). (This brings home the point that it is not possible to speak meaningfully in positive terms about principles like the One: Divine Mind is defined by its not being Soul, the One is defined by its not being either Soul or Divine Mind.)
Because of its unified aspect, Soul participates in the same divinity that characterises the One and the Divine Mind that it moves:
The Soul once seen to be thus precious, thus divine, you may hold the faith that by its possession you are already nearing God: in the strength of this power make upwards towards Him: at no great distance you must attain: there is not much between.
But over this divine, there is a still diviner: grasp the upward neighbour of the Soul, its prior and source. (V, 1, 3)
Here we find something that characterises all of Plotinus' metaphysical philosophy—an emphasis on the necessity of overcoming the world we are part of, and in so doing, moving upward through the hierarchy of Soul and Divine Mind to reach unity with the One itself. In possessing Soul (by virtue of our very existence, since all of nature is shaped by and imbued with Soul) we are capable of grasping "its prior and source", that is, Divine Mind. And with the relationship between Divine Mind and the Soul in mind, conceiving of how Divine Mind relates to the One will perhaps become simpler—just as Divine Mind is the source of the Soul and all the unity it has, the One is the source of all the unity in Divine Mind.
Divine Mind itself can be thought to mediate between the One and the Soul in two distinct ways, one for each direction that the relationship between the One and the Soul can be experienced by either—that is, where the Soul is fallen from unity with the One; and where the Soul is striving to return to that unity. In the former case, Divine Mind is the means through which the unity of the One is filtered down into Soul—Divine Mind is what guides that unity into the multiplicity of Soul. It is unity and multiplicity at the same time—the unity of the One as it is split apart into the multiplicity of Soul. In the latter case, Divine Mind is what allows Soul to come into the ecstatic union with the One that it seeks—intellectual contemplation is the heart of Divine Mind experienced in this way, and it is the only means through which the One can be reached. In either case, Divine Mind is the closest to the One that it is possible to get without actually being unified with it—and in both cases, Divine Mind is the way through which the One is able to exercise influence on the world.
Plotinus. "Enneads", trans. Jason L. Saunders, from Greek and Roman Philosophy after Aristotle. The Free Press, 1994.