Now that the Doctor has control of his TARDIS, Jo can go wherever she chooses. She opts to attend the Campaign for Nuclear Disarmament march in 1960, having heard many stories about it from her father.
Jo is taking this very seriously, and as they march, she worries that the Doctor will say or do something to embarrass her. She is delighted when he befriends Bernard Helmann, a philosophy tutor who is hoping for some lively debate. She leaves the Doctor with his new friend.
Jo meets Joan, a veteran of these marches and the leader of one of the local groups. When they stop for the night at a church, she meets the rest of Joan's people, including a Frenchman named Didier. They discuss the horrors of the hydrogen bomb.
The next day, Jo seeks out the Doctor and tries to convince him to travel back in time to stop the invention of the bomb. He seems uncomfortable and returns his attention to Bernard.
On the third day, Jo and the Doctor have a serious conversation. He helps her to see that it's not that the atomic bomb is good or bad; it's the use of it that is good or bad. Jo finally understands and has found peace with the answer.
On the final day, the marchers reach London, all one hundred thousand of them.
Seventeen years earlier, the Doctor slips into the office of Niels Bohr. He corrects some of Bohr's handwritten notes, ensuring that the atomic bomb project will not be a failure.
- Jo mentions the Doctor interfering in the histories of Sir Walter Raleigh and Christopher Columbus.
- The Bobodix of Lonsee-23 use nuclear devices to "caramelise their snadge puddings".
- Robert Oppenheimer was the inventor of the atomic bomb.
- As possible destinations, Jo considers meeting her ancestors, visiting Mata Hari, visiting the West, or watching a Beatles concert.
- The Doctor acquired his cape in Venice.
- Joan lost her husband in the Blitz.
- Charles de Gaulle is the leader of France in 1960.
- The Doctor altered Neils Bohr's notes.
to be added