Plato was the first philosopher to discover the metaphysical phenomenon of plural-subjects and plural-predication; e.g. you and I are two, but neither you, nor I are two. I argue that Plato devised an ontology for plural-predication through his Theory of Forms, namely, plural-partaking in a Form. Furthermore, I argue that Plato used plural-partaking to offer an ontology of related individuals without reifying relations. My contention is that Plato’s theory of plural-relatives has evaded detection in the exegetical literature because his account of plural-subjects through the Theory of Forms had not been recognised for what it is. I further submit that Plato’s handling of related individuals through plural-predication is not only a “first” in philosophy, but also an “only”, having remained a unique account in the metaphysics of relations. I hope that Plato’s account will introduce a fresh approach to contemporary debates on the subject.