Everything is still speculation as to what might happen in the episode, but the synopsis paints a pretty good picture:
“Someone is kidnapping the Doctor’s friends, leading him towards the one place in all of time and space that he should never go”
The episode takes place in Victorian times again, so it looks like it will have something to to do with Vastra, Jenny, and Strax. Couple that with River being in the episode as well as the Great Intelligence, looks like the GI is kidnapping the Doctor’s friends, and taking them to Trenzalore, for obviously something nefarious.
The episode airs on May 18th, so hopefully as we get closer to the air date, I will have more information (excluding major spoiers) to put on here
