I might be wrong but in order for plagiarism to be established it has to consist of a certain percentage of the whole to actually be plagiarism. I'm not even sure 'plagiarism' is a legal term. Is it just an expression used morally? 'Infringement of Copyright' is surely the legal term and as I said this has to be a substantial percentage of the original work. Certainly with books you are allowed to quote/use a certain percentage in a work of your own without infringing copyright.

Otherwise, if it can be shown that the first line in question existed in the public domain then there would be no problem.