Declining a paper review after accepting it and seeing the manuscript Most of the time, when I am invited to review a paper, I am only provided with the title and abstract. The full manuscript becomes available only after I have agreed to review the paper. As per this question, this seems not to be common in all fields, but it does seem to be the norm in the journals I usually review for.

I assume this is a way to minimize the number of people that get access to the content of the paper before publication, and only provide that "privilege" to those who actually give something back in the form of a review.

However, it might hypothetically happen that I agree to review a paper based on the abstract, but then I see the full manuscript and find that I do not have the required level of expertise to review the methods used in the paper. 

If I now decline the review, then I am going back on my word, and the community will see me as less reliable. Furthermore, I have unfairly gained access to unpublished content without giving anything back.

If I do the review, I will not be able to accurately evaluate the contributions of the paper, and might (probably) recommend acceptance of something that might not warrant it.

What is the best course of action in this case?
The best course is to just inform the editor that you are unable to perform/complete the review as it is outside your area of expertise. The title led you to think otherwise. You are sorry, but others are better qualified to judge and give advice. 

I don't see that as anything strange at all, and no one is likely to think less of you. 

What would be wrong is to give a review of something you don't comprehend completely. No one benefits from that. 

Just be honest with the editor. No one knows everything.