Arriving on a momentous day, 23 October 1843, the Fifth Doctor takes Nyssa and Tegan to Trafalgar Square. He finds several stone lions there that are invisible to everyone but the travellers. To investigate the matter, he decides to climb Nelson's Column. At the top he finds that the column's contractors, Samuel Morton Peto and Thomas Grissell, have been possessed by a race known as the Sevakrill. They reveal that the column has been fitted with a missile that will destroy the Earth in 2017, distracting their enemies, the Charnal horde.
The Doctor urges Peto and Grissell to build a mental barrier between themselves and their host. In the square, the lions chase after Nyssa and Tegan; they kill seven civilians and a policeman in the process. Finally, the Sevakrill are forced out and the lions return to their plinths. The Doctor spends two weeks lowering the missile into a tunnel beneath the column and builds a floor over it. He leaves behind a short range sensor that will warn him should the missile ever be activated. He comments passingly that he might check to see if someone has tampered with Cleopatra's Needle.
- Tegan comments that Trafalgar Square was more "tourist friendly" in 1982.
- Perception filters only work on people from that time. Several Endovorms — with the appearance of stone lions — are guarding the square, made invisible by the filters.
- The Doctor climbs Nelson's Column.
- In the chaos, a Punch and Judy stand is abandoned with the puppeteers still wearing the characters.
- As he leaves, the Doctor decides he wants to "hop ahead 35 years to the embankment", so he can give Cleopatra's Needle a checkover.
- The story is a reference to an old legend that the lions in Trafalgar Square will come to life if Big Ben chimes 13 times.
- The Doctor mentions that he was a personal friend of Horatio Nelson. (PROSE: World Game)
- Tegan recalls the last time that the Doctor climbed something "really tall" and notes that it did not go well. This refers to the Fourth Doctor climbing Pharos Project Radio telescope, being tilted off the gantry by the Master, and falling to the ground, where he regenerated. (TV: Logopolis)