The one where the Doctor inadvertently inspires a major historical event...
No, not that Inferno, not yet. This Inferno refers to the Great Fire of Rome, and there are no planet-boring drills to be seen anywhere.
After episode 3's out-and-out comedy, the tone settles down a bit here, and gets on with the job of resolving the loose ends and getting the Doctor and his friends out of strife and back to safety. The Doctor aka Maximus Pettulian risks being eaten by lions in the circus, Ian is on the run from Nero's guards, and Barbara is constantly at risk of being murdered by a jealous Poppaea. Only Vicki seems to have escaped any direct danger in The Romans!