To answer your literal question first, no, relying on just a single source isn't plagiarism, as long as you correctly cite that source and attribute any quoted or paraphrased material to it, so that a reader can clearly tell which parts of your paper are your own ideas or interpretations and which are merely summaries of what the source claims.
Also, plagiarism actually has very little to do with copyright violation, except that the easiest and most blatant way to commit both at once is to copy-paste a large block of text (or an illustration) without attribution from a source written by someone else and try to pass it off as your own work. But once you get into less blatant examples, it's quite easy to commit plagiarism without violating copyright or vice versa. So your mention of the translation being under copyright is mostly a red herring.
Aside: Copying a block of text or an illustration from another source and attributing it is usually enough to make it neither plagiarism nor copyright violation, at least as long as the copied amount isn't truly and needlessly excessive. But the reasons for this differ. The reason why copying with attribution isn't plagiarism is simply that, by correctly attributing the copied material, you're no longer trying to take credit for it as your own work. The reason why it's usually also not a copyright violation is that copyright law in most parts of the world has a pretty wide exception — either codified into statute law or established as a form of "fair use" or "fair dealing" by courts — for quoting portions of copyrighted works for the purposes of criticism, education or academic research, which a properly attributed and reasonably scoped quote from a source in an academic article about or building upon the source will usually fall under.
All that said, I find it somewhat surprising that merely "taking a 250-page novel […] and reducing it down to about 3 pages", even if bracketed by your "own thesis statement and conclusion", would actually yield a good history paper without relying on any other sources.
Are you really planning on:
- taking your sole source 100% uncritically at face value, and not comparing it with anyone else's account of similar or contemporary events;
- basing your thesis statement and conclusion on nothing but the source text itself, without relying on any other information about the time, place and events covered in it;
- trusting the translation you're using to be 100% faithful and accurate (it never is!) and not comparing it with either the original text or other translations; and
- not contrasting your interpretation of the primary source with anyone else's analysis and interpretation of it?
If not, to any of these points, then there probably are at least some other sources that you should cite. And if your answer to any of the points above is "yes", then I would strongly suggest you at least think twice about it. A good academic paper is generally not supposed to just uncritically digest and regurgitate its primary source material, but to analyze it in a broader context and to illuminate aspects of it that an uncritical modern reader of the original work might otherwise miss, misinterpret or be misled by.
Unless, of course, your "history paper" is really just a "Cliff's Notes" summary of the original text. Although even Cliff's Notes™ generally include some critical analysis and background context.