In a temporary imperial conclave on the road to Parthia in 111 AD, the Tenth Doctor, Gabby Gonzalez, and Cindy Wu are surrounded by Zoroniss. As the leader of the Zoroniss explains their conspiracy to incubate their eggs within Roman soldiers and conquer the world with an army of Zoroniss young, a burst of light flies into the Doctor's back and he falls down. Concerned, his companions console him, but he assures them he's fine and that "whatever it was, it doesn't seem to have had any lasting consequences". They get back to fighting the Zoroniss.
A considerable amount of time later, the Eleventh Doctor and Alice Obiefune watch the first Congress of the United States pass the Tariff Act on 1 July1789. They'd previously been trying to see a musical about Alexander Hamilton, but - out of concern for encountering the Doctor's past selves - decided to visit the real Hamilton instead. The congress is interrupted by an army of French zombie werewolves. As Alice and the Doctor run away from the werewolves and assemble an anti-nanite spear, Alice notes that the Doctor's vocabulary has been strange ever since they landed in Philadelphia. The Doctor thanks Alice for being so considerate and they get back to fighting werewolves.
A 2016 comic convention; in an incredibly short time span, the Twelfth Doctor uses the words condone, congregants, converging, conspiracy, controversial, confiscate, and consolation. Then, he remembers Rome and realises that something has been connected to him ever since. But why only realise this now? The creature hid not just in the Doctor, but also in words that began with "con". The Doctor contacts the creature, a word-rider, and learns its life story. It became stuck on Earth in the distant past and bonded with the Doctor hoping he would take it home. Only with so many people saying the word con at the convention did the word-rider get enough strength to make itself known. The Doctor drops the word-rider off at its home planet, finally ending an incident that spanned three of his incarnations.