Publisher's summary Edit
Responding to a desperate summons from the Doctor's future self, he and Peri find themselves on a sailing ship in 1492, where the crewmen are gripped by superstitious fear. They say the Devil walks among them, stalking and striking them down.
Even though they have landed in paradise, they fear that 'El Diablo' himself will drag them over the edge of the world and into the depths of hell. When the Doctor and Peri meet the captain of the ship, they both discover that heroes can sometimes behave un-heroically. Peri's reaction leads her into deep water, and soon the Doctor fears not only for her life but also for the existence of the ship, the paradise island, and the Universe itself...
The Eleventh Doctor uses the TARDIS's telepathic circuits and viewscreen to contact the Sixth Doctor and Peri Brown. He makes it clear that he is a future incarnation of the Doctor (with Peri at first reflecting that he is what she would expect from the Doctor's son, if he had one), and compliments his previous self; and then he makes a request. He wants the Sixth Doctor to obtain an omniparadox, a most dangerous item. After he leaves, the Sixth Doctor explains that an omniparadox is a sort of power cell, created by the conflict between two versions of time, much as nuclear power is created by smashing atoms together. The omniparadox, however, possesses energies that, if misapplied, can destroy the universe.
The Doctor constructs a device to track the signal of an omniparadox; it does so by mimicking the signal to create a resonance. Tracking, they land aboard a ship—not a spaceship, but a sailing ship—and find the paradox hovering above the TARDIS. However, they are quickly captured by a most unlikely man and his crew, and find that they are in the presence of the famed Christopher Columbus, aboard the Santa Maria; and he has just sighted land. He assumes they are natives of the island he has discovered, and that they have somehow come aboard to worship the invading Europeans. (The fact that he can converse with them without trouble seems to be lost on him.) The misunderstandings are interrupted, however, when it is revealed that a man on board is dying—and claims to have seen the devil.
Unfortunately, Peri has seen it too, albeit briefly. The Doctor gives her the TARDIS key to fetch a medical kit; and en route, she sees a demonic creature in the shadows for a moment. The Doctor determines that the man is dying of tuberculosis; he has the ability to cure him, but refuses to do so, as introducing modern medicine to the year 1492 could be disastrous. Enraged at him, Peri runs off through the hold where the TARDIS is parked, stopping only to throw the key at the Doctor.
Peri comes to the prow, intending to just stand there and think, but falls overboard. The Doctor panics, and tries to enter the TARDIS to save her, but cannot find the key. He is diverted, however, when he sees that the omniparadox is now gone; and shortly thereafter, the universe begins to violently unravel. The Doctor realises that something has caused the paradox to be removed, which means that the Eleventh Doctor's mission in the future will fail, bringing about this destruction; but he stabilises the situation briefly with his tracking unit, using its false signal to "trick" the universe into stability. It will not last, however, and he has about an hour before things fall apart. Columbus, having had his beliefs challenged repeatedly, now believes the Doctor is a wizard, and orders him to find the key and fix the situation; if he does not do so in twenty minutes, Columbus will cut off his hands, a punishment that history attests he used often on the native populations.
Peri, meanwhile, is not dead. She finds herself washed up on the shore—and is immediately captured by natives who are under the control of a monster. The monster is the devilish figure she saw; it confronts her, and reveals itself to be the Herd Leader of the Bovine race, a race of intelligent buffalo. Once they ruled the continent, and the primitive humans worshipped them; but then the herd leader was trapped in ice. Without its mind, the herd regressed into common buffalo, and were hunted to extinction. In the future, when the herd leader thawed out, he found he had no herd to lead. Adopting time travel technology which had since been developed by humans, he travelled back to conduct experiments which would save his people. He believes that Peri and the Doctor were sent to stop him.
The Doctor determines that a goat in the hold has eaten the key. However, he retains a psychic connection to it; and he is able to telepathically connect it to the TARDIS despite the goat (and much to the goat's alarm) and get the door to unlock. With Columbus in tow, he determines that Peri is alive, and travels to her location; unknown to him, Columbus—now convinced the Doctor is a superior explorer—plans to kill him out of jealousy.
Arriving at the Herd Leader's time machine, they learn its plan. It was the herd leader that led Columbus to the new world—Columbus being an incompetent navigator on his own—in hopes that the Europeans will exterminate the native Americans, thus preventing them from exterminating the Bovine herd. In that way he can return to the future and resume his place as herd leader. They are shocked to see another Herd Leader appear and interrupt, however; or rather, the same one, but older. The second leader says he is from the future, and has come to stop the experiment, because it will be a failure—the Europeans, too, will hunt and control the Bovine. The Doctor uses this opportunity to surreptitiously remove the time element from the machine. Warned by Peri, he dodges out of the way as Columbus tries to kill him with a sword; Columbus misses and destroys the time element by accident. The second herd leader vanishes, being unable to have time-travelled without the machine; the first is forced to flee. After removing the time machine, the Doctor, Peri, and Columbus return to the ship.
Columbus is forced to acknowledge that the Doctor and Peri are not natives after all; this does not change his plans, but he debates recording these events. He sends his men ashore to hunt down and kill the herd leader, convincing them it is not a devil, but an animal. The Doctor sees that the omniparadox has returned, and collects it; he theorises that it disappeared because of the likelihood of Peri's death. Without her to warn him of Columbus's strike, the timeline would have been vastly different; and it was the collision of the timelines of the two herd leaders that created the paradox in the first place. Having a final change of heart, he cures the man with tuberculosis, and then they depart.
- Peri's father Paul Brown owned a Chevy, which she describes as a "jalopy."
- The Eleventh Doctor tells his sixth incarnation to obtain an omniparadox for him. He tells his younger self that he will need it in the future to save the universe.
- The Eleventh Doctor claims that his TARDIS has a GPS with the voice of Davros.
- Christopher Columbus believes that Peri is an offering to him from the natives.
- Columbus' ships are the Santa Maria, the Niña and the Pinta.
- Peri was a straight-A student at high school.
- By the 15th century, it was against Spanish custom to engage the slave trade.
- The Doctor wears cat cufflinks.
- The Bovine Herd Leader, whom Peri describes as a "space buffalo," claims that bovine horde ruled the Earth in the distant past and humans worshiped them as gods.
- The Eleventh Doctor refers to his fez. (TV: The Big Bang, et. al)
- Peri refers to her encounters with George Stephenson in Killingworth in the 19th century (TV: The Mark of the Rani), H. G. Wells on Karfel (TV: Timelash) and Richard III in 1483. (AUDIO: The Kingmaker)
- Peri mentions that she is from Maryland and that her mother is Janine Foster. (AUDIO: The Reaping)