Doctor Who: Series 13 Trailer Breakdown - 20 Things You Must See
11. The Crimean Capture
Remember how we said that the special effects are terrific? Well, this green planet is the exception. It's not terrible - it's just not as good as everything else.
Whatever this planet ends up being called, this entire sequence - including the above shot of a captured Doctor and Yaz - might take place in the same episode as the Crimean War. See that red shirt/dark jacket combo Yaz is rocking? She has that exact same outfit on when she meets Mary Seacole earlier in the trailer.
Now yes, it's possible that Yaz wears the same outfit across multiple episodes, especially considering that this is a serialised story. But typically, companions wear one costume per adventure, and if that's the case here, then this is our theory: the green planet is the Crimean battlefield at a future point in time.