WHO is Dr. Who?

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Can someone please explain Dr. Who to me? I like sci fi and time travel, and I have heard that the Dr. Who series is about those topics. I also heard about the two different versions, the older and the more recent versions. I see it on Pluto on my ROKU but on there you don't start at the beginning and don't always see full series episodes. Is there any way to get the original series in streaming on ROKU in order? Or even the new version? And is the new version fully complete without needing anything from the old version to make sense of it?