Should a paper be reviewed in entirety, if the introduction contains plagiarized passages? I'm very new to being a peer reviewer. I agreed to anonymously review a paper for publication, and while reading it for the first time, I was a little annoyed by the writing style. It sort of reminded me of the feeling I get when reading a paper from a student in an undergraduate Liberal Arts Math course. When I began to read it again, I was very uneasy about the writing, especially in the introduction. There were inconsistencies in style. Some short, dry sentences followed by longer passages using flowery language.  I looked at the references and noticed a few secondary sources. One was a NY Times article. I looked up the article online and in the first paragraph found a passage that was almost identical to one in the introduction section of the paper I'm supposed to review. I was shocked. And then I found more. 

So far, all of the plagiarism that I've found is in the introduction. I haven't read the rest of the paper carefully yet because I'm fairly disgusted. 

My question is this: should I even bother writing a review? If this were an undergraduate paper, the student would get an F on the assignment and get reported to the Dean. I want to write to the journal editor and just tell him that the paper doesn't deserve to be reviewed. 

Has anyone seen this before, and what did you do? If you decided to review a paper like this, how would you phrase your feedback?
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.