At the top of the Platoinian ontological scale there are three hypostases: the One, the Intelligence and the Soul. Each of them possesses certain characteristics and relationships amongst themselves and with the multiple entities of the world; as it is easy to foresee, at the top of this scale presides the One.

The One is all things and no one of them; the source of all things is not all things; all things are its possession- running back, so to speak, to it- or, more correctly, not yet so, they will be. 
[Plot. Enn. V, 2, 1][1]

From this step we can take further steps to fully understand what the One of which Plotinus speaks is. 
The One, as the principle of the Whole, is not identifiable as the whole of all things, i.e. at the beginning of the movement (it is allowed to use these terms only in a broad sense, since temporality is not yet present here, nor would it be possible to discuss whether this ‘movement’ is only logical or also concrete) the ontological principle is different from the ontological totality. At the same time, the One will be the Whole because there is a ‘movement’ of return of all things towards unity. But how is it possible, first of all, that multiplicity emerges from unity?

It is precisely because that is nothing within the One that all things are from it: in order that Being may be brought about, the source must be no Being but Being’s generator, in what is to be thought of as the primal act of generation. Seeking nothing, possessing nothing, lacking nothing, the One is perfect and, in our metaphor, has overflowed, and its exuberance has produced the new […].
[Ibidem]

This passage, which is really complex, requires a wider explanation. The question that was asked just now was how it is possible to pass from Unity to multiplicity; Plotinus therefore begins to demonstrate how the One is generative in itself. In the ontological principle the Whole is somehow present – even if not in the form of totality understood as the ‘sum of all things’ – therefore it is evident that everything can, potentially and effectively, derive from the One. The term ‘deriving’ here indicates the ‘being’ of something, and in this sense, everything derives its own being from the One. And in this way also the Being itself, understood as the existence of all things, derives in turn from the One. This is a point of extreme importance: if we understand Being as precisely ‘the existence of all things’, it is evident that the One cannot be the Being, because there would be identity between the being of all things and the One, and therefore the latter would no longer be the ontological principle of all things, but the being of all things, and therefore properly the ontological totality. The One is the parent of the Being (which of course is the pure Being, since there is still nothing of which one can preach being), and this is the first geniture of Pleninian hypostasis. In this sense, the One is prior to the being of all things, and therefore seems to be in a position prior to the Being itself. The One’s perfection – he seeks nothing because he needs nothing and because everything else is subsequent to Him – corresponds to a generative superabundance; i.e. it is the cause of his generativity (remember how this also happens for the Platonic Good). Having clarified this point, attention can be drawn to the second of the hypostases.

[…] this product has turned again to its begetter and been filled and has become its contemplator and so an Intellectual-Principle [νοῦς]. That station towards the one [the fact that something exists in presence of the One] establishes Being; that vision directed upon the One establishes the Intellectual-Principle; standing towards the One to the end of vision, it is simultaneously Intellectual-Principle and Being; and, attaining resemblance in virtue of this vision, it repeats the act of the One in pouring forth a vast power [δύναμιν].
[Ibidem]

The Being generated by the superabundance of the One naturally turns towards the latter (since it can only turn towards its generator), and this ‘looking to’ corresponds to a filling – which is also a fulfilment and a satisfaction, since the verb used by Plotinus is πληρόω. At the same time a self-reflexive movement happens, and the Being turns towards itself; this self-reflexivity is referred to Intelligence according to Plotinus (and in fact by intelligence we also mean in a wider sense a sort of self-awareness). It is interesting to note that, after the generation that takes place thanks to the superabundance of the One, the distinction and stability of what is generated depends on a’ looking to’, and therefore from a relationship: Being is linked to a reflexive and self-reflexive relationship, and for this reason there is a form of identity between Being and Intelligence (between Being and rationality in the broadest and highest sense of the term).
The contemplation of Intelligence presupposes being (being) near oneself, that is a form of ontological stability, and for this reason it becomes both Intelligence and Being. Just as the One, so the Being generates what is similar to it, and in order to do this it pours out its power; this generativity is nothing but an image of the superabundant and original generativity of the One. 
At this point, although we have reached the second hypostasis and the bases of existence understood as ‘the being of what is’ have been laid, we have not yet answered our first question: how does the passage from the One to the multiple take place? To find out, it is necessary to continue with Plotinus’ text.

This second outflow is a Form or Idea representing the Divine Intellect as the Divine Intellect represented its own prior, The One. This active power sprung from essence [from the Intellectual Principle considered as Being] is Soul. Soul arises as the idea and act of the motionless Intellectual Principle – which itself sprang from its own motionless prior- but the soul’s operation is not similarly motionless; its image is generated from its movement. It takes fulness by looking to its source; but it generates its image by adopting another, a downward, movement. This image of Soul is Sense and Nature, the vegetal principle.
[Ibidem]

The generative power of Being (in this case force or energy because the term used is ἐνέργεια), which proceeds from Being towards the outside is called by Plotinus ‘Soul’ [ψυχή], and, unlike the One and the Intelligence, it becomes. Basically, the birth of the Intelligence corresponds to a self-reflection of Being in its immobility – and for this reason the Intelligence is in turn immobile – whereas the Soul is an outpouring of generative power of Being outside itself, and therefore it is in motion.
This movement, which we could in a certain sense define as ‘motion of the Soul’, leads it to generate its image, and is once again equivalent to a generative ‘turning to’. When it turns to the Intelligence, it generates thought; when it turns to itself, it establishes its conservation, its stability; when it turns to what is lower than it, it becomes an ordering force, governor and regent. In this way the Soul, turning towards Intelligence, orders the universe as thought; at the same time, turning downwards, it governs the corporeal universe, and it is Providence. 
However, a doubt remains: when the Soul turns downwards, what does it govern? Obviously the corporeal world, which is manifold, imperfect and, at times, evil. Are we to understand that these out of tune, these out-of-tune notes are generated by the Soul? Absolutely not. Plotinus introduces at this point the very principle of imperfection: matter. It is nothing but a deprivation of reality and good, the maximum distance from the One, a material and inadequate element that is informed by the soul’s ordering spirit.

Summering up what has been said, the three Pleninian hypostases are the One, the Intelligence and the Soul. They are closely reminiscent of the Christian trinity, but it is necessary to look at undue assimilation of concepts. The One is not a rational and creative God, but a generative ontological principle. Intelligence is the self reflexivity of Being itself: when Being turns to its creator it derives its ontological stability, when it turns to itself it becomes Intelligence, that is, reflection of Being on Being. The Soul is the power of Being that becomes Thought and Providence that governs matter, that dark shadow of evil and imperfection that appears at the limit of light.
The grandiose Plato’s construction is particularly interesting because within the ontological movement the relationship, the ‘turning to’, finds space, and marks a fundamental and essential step for all subsequent philosophy.