It could be in Rebels or it could be in one of the movies or spinoffs... anyway: A world at roughly our present level of technology. Almost all the planets we've seen, no matter how backwater, have some form of contact with the galactic community at large. About the only society that didn't was the Ewoks... And in fact in RoTJ we basically saw the end of that, as the alliance made first contact with them. But there must be countless pre-starfaring planetary civs out there at all levels of development, and with which no galactic concerns have yet made overt contact. Up to and including ones like ours - post industrial, information technology societies, with primitive space tech (e.g. We today have the ISS, had a brief lunar exploration from 1969 - 1972, have exploratory probes throughout the solar system and thousands of sats in Earth orbit). Such societies could remain blind to the galactic community as long as they depended on basic EM radio for communications, having no ability to receive subspace and hyperwave s-thread transmissions. For example, a program like SETI would yield nothing because the galactic community has progressed beyond the primitive speed-of-light EM comms that said program looks for. There's a couple of ways this could be handled... Either as an undercover op where some group had to insert among said society, while keeping their status as an offworlder secret, or as a contact situation, in which we bear witness to that society becoming aware of the wider galactic community and the existence of "aliens" (from their POV) for the first time. Well, I guess such an ep / arc might need a careful hand to avoid it becoming too Star Trekky in look and feel, but I think it could be done all right.