Plotinus' relation to Plato seems straightforward but is not. But that he has some relation to Plato, that he treats Plato frequently as a touchstone, is clear. Jewish influence on Plotinus' thought, via Philo, is a possibility for which I can only give references.
Plotinus and Plato
E.R. Dodds sets out the seemingly straightforward relation as follows :
It is natural to begin by asking what Plotinus thought of his own work and how he
conceived his historical function. To this the answer is easy, but disappointing. Plotinus
apparently did not know that he was a Neoplatonist; he thinks of himself as a Platonist
tout court. ' These doctrines,' he says (5, 1, 8, 10), speaking of his own system, 'are no
novelties, no inventions of to-day; they were stated, though not elaborated, long ago;
our present teaching is simply an exposition of them-we can prove the antiquity of these
opinions by Plato's own testimony.' This is not the language of a creative thinker acknow-
ledging his debt to a great predecessor ; it is the language of a schoolman defending himself
against a charge of unorthodoxy ... And indeed Plotinus avoids as a rule making
any claim to originality; the rare exceptions have reference only to details of his system. (E.R. Dodds, 'Tradition and Personal Achievement in the Philosophy of Plotinus', The Journal of Roman Studies, Vol. 50, Parts 1 and 2 (1960), pp. 1-7 : 1.)
Dodds next proceeds to look past first appearances - the 'easy answer' - and to vindicate Plotinus' originality :
Formally, but only formally, the philosophy of Plotinus is an interpretation of Plato;
substantially, I should call it an attempt to solve the spiritual problems of his own day in
terms of traditional Greek rationalism. He nowhere openly disagrees with his Master,
though he recognizes that Plato sometimes speaks in riddles (6, 2, 22), leaving us to work out
his meaning for ourselves (5, 8, 4), and also that his teaching is not always consistent, at any
rate on the surface (4, 4, 22 ; 4, 8, 1). For each of the major features of his own system he can produce, and feels obliged to produce, certain Platonic texts as 'authority' ...
But these Platonic texts are not the true starting-points of his philosophy: he does not
believe in the One because he has found it in the Parmenides; on the contrary, he finds it
in the Parmenides because he already believes in it. Nor does his exposition normally start
from Plato: his more usual method is to state a problem and try out various ways of
solving it until he arrives at something which he finds logically satisfying; then,
and most often only then, he will cite for confirmation a text from Plato. In fact, he quotes
Plato pretty much in the same spirit in which some seventeenth-century philosophers quote
Scripture-not as part of his logical argument but as evidence of orthodoxy. His basic
question is not the historical one, ' What did Plato think about this ? ' but the philosophical
one, ' What is the truth about this ? ' Respect for the Great Founder required, indeed, that
both questions should have the same answer. But where violence had to be used to achieve
this agreement it is generally Plato who is wrenched into concordance with the truth;
so far as I can judge, the truth is seldom distorted to make it agree with Plato. Had Plato
never lived, Plotinus would have had to formulate his thought in some entirely different
way, but I am tempted to guess that its general structure and direction would still have been
recognizably what they are. What spoke to him in Plato's name was his own daemon, even
when it used the very words of Plato. (Dodds, 1,2.)
Dodds recognises that 'Stoic and Peripatetic elements are to be found in his [Plotinus'] writings' (Dodds, 2) but what of Plotinus's possible indebtedness to Jewish thought ?
Plotinus and Philo
Both McGinn and Idel have noted the importance of mystical union in
Philo’s thought and its possible influence on the articulated discussions of
unio mystica in Plotinus and consequently on the entire Western mystical tradition ... Bernard McGinn, “Love, Knowledge, and Mystical Union in Western Christianity: Twelfth
to Sixteenth Centuries,” Church History 56 (1987): 7–24, and “Love, Knowledge and Unio Mystica
in the Western Christian Tradition,” in Idel and McGinn, Mystical Union in Judaism, Christianity and Islam, 59–86. (Adam Afterman, 'From Philo to Plotinus: The Emergence of Mystical Union', The Journal of Religion, Vol. 93, No. 2 (April 2013), pp. 177-196 : 178.)