The Master's plans have gone wrong. He has been in Chicago for two months. He had gathered the shards of the Godhead and made a deal with Havuri-sss-Cazcim of Blue Profundis. He knows where the Thought Core is. However, his TARDIS has been stolen, so now he is stuck on Earth.
As Sam Kulisa drives the Master, known as "Dook Domini," to his hotel, they notice that there are few police in the streets. When they reach the hotel, they understand why. There are police waiting outside the hotel entrance, so the Master sneaks in through the kitchen. In his rooms, he finds his secretary, Margie del Monsalvat, talking with police chief Mulligan but not giving him any answers.
Mulligan questions the Master, but gets little information. The Master suggests that Mulligan is in the pay of his rival, Joe Clementi, but instead of offering more than Clementi pays, hypnotises Mulligan instead.
After Mulligan leaves, the Master receives a message from Havuri-sss-Cazcim, who accuses him of deceiving his people. As the Master and Margie watch the transmission, a star-like light approaches Havuri-sss-Cazcim's location and destroys the planet.
Margie drives the Master to the Wainwright Museum and learns what he's been doing the last two months: he's been trying to figure out how to steal a chunk of tekrite, which is actually the Thought Core. They are accosted by Wainwright, who explains to them the nature of the Godhead and its dangerous power. She is from the future, the end of time, and her people destroyed the Godhead to prevent its use. Her purpose is to guard the Core. When the Master tells her he has gathered the shards of the Godhead and intends to unite them with the Core, she tells him the Core is self-aware and is using, not just him, but also Margie, who has been coming to the museum before the Master started doing so.
Suddenly, Margie shoots Wainwright with the Tissue Compression Eliminator, which the Master thought was in his stolen TARDIS. Clementi and his men arrive, and the Master learns Clementi is working for Margie. Clementi's men grab the Master, but he is able to escape. He leaves the museum, but he is mugged, losing his money and his watch. He did, however, manage to grab the armilla, the device that Wainwright had been using to keep control of the Core.
The Master ends up at the Mission Hall, where he spends the next four days helping to make soup and keeping silent. On the fourth day, hearing a voice in his head, he goes back to the museum and climbs up to a window on the first floor. Inside he sees Margie, Clementi and his men trying to get the Core. They spot him and try to kill him, shooting him as he runs away. He notices a star-like light approaching the museum, and realises it's his stolen TARDIS, under the control of the Core. It starts shooting at Clementi's men. The Master runs back to the Mission Hall and retrieves the armilla from his mattress. It is starting to sweat mercury, so he needs a container for it.
He finds a phial of holy water and empties it. As he is letting drops of mercury fall into the phial, Father Sherrin walks in on him. He allows the Master to take the phial, saying the holy water is fake, and gives him the watch that he lost, but the Master kills him and leaves. Some of the mercury lies on the floor next to Sherrin's body.
As the Master leaves the mission, he sees people running in panic. The statue of Abraham Lincoln from the museum is walking down the street towards him, Margie riding on its arm and carrying the Core. Clementi and his men are behind them. The Master's TARDIS also arrives. When they reach the mission, Margie tells the Master that the Godhead has a failsafe — it needs a mind to control it, and it wants his mind.
The Master runs back inside as the statue smashes parts of the building looking for him. It grabs him, and he sets his watch to emit sound at a frequency that destroys the statue and part of the building. As the Master is buried under debris, he hears a horrifyingly familiar sound. The Doctor's TARDIS materialises on top of him, and though he doesn't recognise the voice, he knows it's the Doctor and one of his companions. They look around and discover the phial of mercury, which the Master had hidden in a coffee pot. It turns out to be just what they needed, as the TARDIS needs mercury for the fluid link. They take the mercury and leave.
The Master is confronted by Margie and Clementi. However, the animated body of Sherrin appears, covered in mercury. Now in control of the Godhead and the Core, he flies out into the sky and away from Earth. The Master heads for his TARDIS and leaves.
- Fourth Doctor
- Sarah Jane Smith
- The Master
- Sam Kulisa
- Margie del Monsalvat
- Chief Mulligan
- Joe Clementi
- Father Sherrin
- At the mission, the Master makes soup with carrots and turnips.
- The Master's men make alcohol out of potatoes.
- The Master is familiar with the Ministers of Grace.
- The Duke of Dominoes was the trump card in an old Wallarian game of chance.
- This story primarily focuses on the Master.
- The Fourth Doctor and Sarah appear very briefly and only at the end of the story.
- The stories in Decalog are linked by the story Playback, in which psychic Silverman examines various items in the stranger's pockets. For this story, he examines the phial of mercury.
- This story is initially dated to the 1930s, then mention is made of Al Capone's recent tax evasion, which happened on March 13, 1931.