Should a paper be reviewed in entirety, if the introduction contains plagiarized passages?
I think you've pretty much done your review.  You found a whole bunch of plagiarism up front, and that's enough to recommend rejection of the article.  Document your findings and report to the editor.  Even if the rest of the article turned out to be brilliant and original, there is no way it can be anything other than rejected and possibly even formal proceedings against the authors.