When we left our review of Foundation, the Foundation had set up religious control over a large area of the galaxy. They were apparently the most powerful government in the galaxy at the time, and everyone who used atomic power got it only through the "miracles" provided by the Foundation’s religious system. But eventually that control will crack.
It begins to crack in what was originally a new book, but now is part of Foundation. The Mayor is trying to use the same kind of control on the Foundation itself that is used to run the other planets. He starts by deciding to destroy a Trader, one of the commercial leaders of the Foundation (and notoriously anti-religious). We first see the Mayor offering Hober Mallow, the Trader, a position in the government — which Mallow refuses. Then another Trader, Jaim Twer, finds Mallow and offers him leadership of an opposition political party. Mallow suspects collusion, since neither of these men much liked him in the past, and turns the tables by letting the Mayor talk him into going off on a secret mission to Korrell (a distant planet that is not completely under the Foundation’s control), then talking Twer into accompanying him on this trip.
It is an exciting trip. They get out to Korrell and land a long distance away from any major city. For five days nothing happens. Then, suddenly, Mallow discovers a man has been admitted to the ship — a Foundation priest, dressed in full priestly garb. But Mallow says they will have to return the priest to the Korrellian authorities, since Korrell’s treaty with the Foundation prohibits priests on their planet.
A crowd suddenly gathers. Things are getting tense in Mallow’s ship. He sends people away on various tasks, and is alone in the main hall for a few minutes. Then he returns the priest to the Korrellians, after facing down his crew at gunpoint. The priest has cursed the crew with wild gesticulations and all the typical priestly curses. Nonetheless, he is expelled from the ship.
Approximately half an hour later, Mallow and his crew are invited to meet the ruler of Korrell. We know, from behind-the-scenes conversations, that Korrell intends to attack the Foundation. Nonetheless, Mallow and his crew sell the Korrellians non-religious atomic devices, including kitchen appliances, industrial equipment, and beauty supplies.
Mallow grows rich on trade with Korrell. He becomes more powerful as a Trader and is a force in Foundation politics. Then the old Mayor decides it is time to spring the trap he laid by sending Mallow to Korrell, and has Mallow arrested for treason.
For several days the prosecution has a great time showing all the reasons Mallow must be a traitor. Their keystone is the incident in which Mallow expelled the priest on his first visit to Korrell. By the end of the prosecution’s case, it looks like Mallow may not have violated any laws, but will be either convicted or lynched. But he has an ace up his sleeve.
When it is time to defend himself, Mallow says that the actions described were as the prosecution has stated, but the reasoning was not. He knew that Twer was working for the Mayor, and that was why he brought the man along on his trip. He provided proof that Twer was actually trained as a priest, not as a Trader.
Mallow pointed out some interesting inconsistencies in the situation he was faced with. First, after no contact for five days, there is suddenly a priest there — illegally. Then, the priest is in his full regalia, but has not been arrested. Within a few minutes of the priest entering the Foundation ship, a crowd gathers. But the nearest major city is two hours away by aircar. So Mallow was also convinced that the priest on Korrell was a fake.
Then he reveals what he did when he was alone in the hall for a few minutes. He had set up a videocamera, and flooded the cabin with ultraviolet light to see what it would reveal. Mallow plays back the scene in which he faces down his crew and the fake priest curses them.
The first point Mallow makes is that Twer shows his priestly education in his reactions to the fake priest. But that is not the telling point. The most exciting part comes when the fake priest was gesticulating, and his arm was bared. There was a flash, as he had a secret tattoo on that arm, which is only visible when illuminated by ultraviolet light. The tattoo is "KSP," which stands for "Korrellian Secret Police."
By this time Mallow has the crowd in the palm of his hand. When he shows that he was acting not only legally but intelligently, and exposes the plot perpetrated by the Mayor, he is acquitted and made a hero by the population. In the next election, Mallow becomes Mayor.
Soon Mallow is thought of poorly again. He has broken off trade with Korrell, but only because they declared war on the Foundation. Through political maneuvering he has become High Priest as well as Mayor, virtually invincible until the next election, but many people only see that he has been dealing with Korrell, who is not attacking, and he is not trying to fight. He believes that the best course is to do absolutely nothing, because Seldon planned to force the Foundation into particular situation that would be solved by great historical forces rather than by intelligent action.
Eventually, Mallow is proved right. Since trade has stopped, all the neat, useful atomic gadgets on Korrell are starting to break down. Soon the complaining of the average Korrellian becomes loud enough to make the leader of Korrell withdraw his ships, and the Foundation wins without a shot being fired.
Mallow then points out that two things caused the Foundation to win: the Seldon Plan and the prevalence of trade. Since the Seldon Plan says that the Foundation will always win, the Korrellians were very timid in their attack, and took much too long. If they had attacked all-out as soon as they declared war, they probably would have won. But the trade between the two planets made Korrell more dependent than they thought on the Foundation. Mallow realizes that this means the era of religious control is ending and the era of control through trade is beginning.