Personally, I would not put a strong focus on the bachelor thesis. Mention briefly what you did, the methods you may have learned and then talk about transferable experience, like attending research group meetings and discussing your results, insights into how to approach your own project, how to acquire samples/subjects etc... there is always SOMETHING. Your audience will not be too interested in the specifics of your project if it is unrelated.
Then I'd talk about how you progressed during your master's what subjects you chose and how your interests evolved. Anything that drew you to the topic you are applying to. And then I'd talk about the planned thesis. Even if you have not started it yet, you probably have a plan what it's going to be? Talk about the background, what you expect to find, what you expect the difficulties might be. In the end, that not only introduces your research topic but also that you have foresight and planning skills.