Thanks to his TARDIS arriving during the second part of Friendship is Magic (AKA, the second part of the first season premiere), the Doctor has been brought to deal with a problem being conducted by the local changeling swarm, though, not before meeting a new companion, a rogue changeling not interested in doing Chrysalis' bidding in the slightest.

As they venture on, they will have to deal with not only Chrysalis, but possibly another dark evil, as well...