Is it crazy to change fields between MSc and PhD?

My field of studies is relatively interdisciplinary. I first got a Bsc in the "humanities" side and then a Msc in the "sciency side" because I had decided to branch out and try something more technical. Since then, however, I realized that I'd want to go back to my original love so to speak and pursue a PhD in that area. There's an interesting opportunity right now.

However, I am completely unfamiliar with the literature in that field. I don't know the professors, the hot topics, and how to build a project proposal. I only did it for my Msc thesis. But I still love the idea of researching on that topic. Would it be irresponsible to try and do it anyways?