The Doctor and his companions travel back in time to when the Romans controlled England. They had not yet conquered Scotland as the Picts was able to put up a fight and prevent them. You can find out more information from a factual point of view by visiting Ref: BBC Bitsizes
The Doctor with Nardole and Bill Potts must try and put the hatred of the Romans and Picts to one side in order to find out what happened to a Roman Legion of 5000 troops. Bill Potts gets separated from the rest of the group and befriends a group of Roman Legionnaires. The Doctor on the other hand helps the Picts.
It soon becomes clear that there is a portal which creatures coming through that are responsible for the kidnappings. The Doctor sets about to close the portal and save the day.
The missing 5000 legionnaires is based on a real mystery that happened thousands of years ago. Here's a link to a page which has information of the missing Ref: BBC.