The storyline for City of Death was devised by David Fisher but was heavily re-written by script editor Douglas Adams, aided by producer Graham Williams, and was broadcast under the pseudonym "David Agnew". It was the first Doctor Who serial to film on location outside of the United Kingdom. The fourth episode was watched by over sixteen million viewers, the highest audience ever attained by an episode of Doctor Who.
While leisurely enjoying the city of Paris with Romana, the Doctor feels effects of time distortion. At the Louvre while admiring the painting of the Mona Lisa, he encounters the Countess Scarlioni wearing an alien bracelet used to scan security systems. The Doctor and Romana meet Inspector Duggan who has been tailing Count Scarlioni for some time, as he has placed a large number of lost art treasures on the market, and fears the Scarlionis are looking to steal the Mona Lisa. Though the three are briefly captured by the Countess, the Doctor helps them to escape and explore the Count's mansion, where they discover equipment by Dr. Kerensky to experiment with time, the source of the Doctor's time distortions. They also discover, behind a wall, six exact copies of the Mona Lisa, each painted by Leonardo Da Vinci himself.
Leaving Romana and Duggan to continue to investigate in the present, the Doctor uses his TARDIS to visit Leonardo's workshop. The workshop is empty but he runs into Captain Tancredi, whose appearance is the same as Count Scarlioni. Tancredi reveals he is Scaroth, the last of the Jagaroth race, stranded on Earth and fragmented through time due to an explosion of their spacecraft on Earth 400 million years ago. Seeking to restore himself and his race, Scaroth has had a hand in aiding human technological advancement to the point of being able to travel through time. Tancredi, in this era, has convinced Leonardo to paint 6 copies of the Mona Lisa such that when Scarlioni steals the real piece, he can then sell it seven times over, substantially funding the completion of Dr. Kerensky's work. The Doctor escapes from Tancredi, returning to Leonardo's workshop. He uses a felt-tip marker to write "This is a fake" on the six blank canvases and leaves instructions for Leonardo to paint over the text, as to allow them to track the copies in the future by x-raying them.
The Doctor returns to the present, and learns that Scaroth has succeeded in stealing the Mona Lisa. Furthermore, Scaroth has killed Dr. Kerensky and threatens to do the same to the entire city of Paris if Romana does not complete Kerensky's work. The Doctor manages to convince the Countess to help them by revealing the Count's alien face, but the Count kills her before she can react. With the time equipment fixed, Scaroth uses it to travel back 400 million years in hopes to stop the explosion of his ship. Though Romana reveals that the equipment will bring him back after 2 minutes, the Doctor asserts that it is enough time for Scaroth to stop the explosion of the Jagaroth ship, itself the source of the the spark of energy that created the beginnings of life on Earth; should Scaroth stop the explosion, humanity will cease to exist. The Doctor, Romana, and Duggan race to the TARDIS and travel back to intercept Scaroth. Duggan punches Scaroth unconscious before he can stop the ship. Scaroth's body returns to the present as the ship attempts to take off and explodes, assuring the development of life on Earth.
In the present, Scaroth is killed by his henchman Hermann, and a failure in the time equipment sets the mansion ablaze. By the time the Doctor, Romana, and Duggan arrive, the original and 5 of the 6 copies of the Mona Lisa has been destroyed, but one of the copies remains untouched. The Doctor proposes that since a copy of a painting by the original artist is not a fake, the painting should be considered the real work of art, reminding Duggan that art is worthless if its monetary value is all the matters. The Doctor and Romana say goodbye to Duggan at the Eiffel Tower.