Make sure that you have read the Faculty information on plagiarism, and that you understand what the university considers to constitute plagiarism. If you are in doubt, check with your supervisor. Plagiarism does not just refer to the deliberate act of passing off other people’s work as your own: you may also be guilty of plagiarism if you do not reference your work carefully enough. Examiners should be in no doubt as to which parts of your dissertation are your own original ideas and which belong to someone else.
In broad terms, any idea (whether directly quoted or paraphrased in your dissertation) which is part of a critic’s argument or interpretation of a text should be acknowledged as such. You do not need to reference facts about the text’s plot mentioned by a critic or any information which may be regarded as common knowledge, such as historical events.