Lieutenant Edward Fyne, a young British officer in the Colonial Police Force, receives reports of a man-eating tiger in the northern regions of Andhra Pradesh and sets off to investigate with his second-in-command, Sergeant Singh. While setting a trap for the animal, they meet a stranger named the Doctor who claims that the “tiger” is in fact a chameleonic alien from another world. Fyne scoffs and sends the Doctor packing at gunpoint, but that night, while waiting for the tiger to arrive, he and Singh are attacked by creatures like hybrids of man and tiger. Fyne’s bullets seem to bounce right off the leader of the pack, but at the last moment, the Doctor arrives and uses fire to drive it off; its inertial force field protects it from fast-moving bullets, but not from the slower, gaseous flames.

The creatures retreat, taking Singh with them, and the Doctor warns Fyne that his friend is doomed to become another of the hybrids. The original alien must have used a tiger for the template for its new form on Earth, and it is now capturing humans and using nanoviruses to transform them into hybrid slaves. It must be stopped before it spreads the infection any further. Fyne enlists the help of the local villagers, but, despite the Doctor’s hope that he can cure the Tyger’s slaves, the villagers fear what their friends have become and gun them down as soon as they find the Tyger’s lair. The villagers kill the Tyger itself and destroy its spacecraft, but when Fyne attempts to rescue Singh, the mutating Singh attacks Fyne and passes on the infection before he too is shot and killed.

The Doctor convinces the villagers to let him take Fyne away to receive help, takes the young man into his TARDIS and makes a hasty jump away from Earth to ensure that the infection will not spread if Fyne loses control and tries to break free. He then sets up a time loop within Fyne’s body in order to prevent the nanoviruses from mutating him any further until the Doctor can get him to the hospital worlds of the Flower Culture. However, the Doctor then discovers that his TARDIS has become trapped in orbit around a black hole, a million and a half years in Fyne’s future. The Doctor has visited this black hole before, in another incarnation; billions of years in the future, it will be the home of the Conservers, a race who will preserve the culture and knowledge of humanity during the last days of the Universe. Presently, however, a generational starship carved out of an asteroid has become trapped in the black hole’s gravity well, and is orbiting about the black hole so quickly that its relative time has become dilated -- and the time dilation is on the exact same frequency as the time loop which the Doctor has set up within Fyne’s body, causing the TARDIS to become stuck here.

Fascinated by this incredibly unlikely coincidence, the Doctor materialises aboard the ship to investigate. Fyne accompanies him outside to find himself inside a hollowed-out asteroid five miles long. However, the inhabitants of the ship prove unfriendly, and the Doctor and Fyne are captured by villagers who apparently assume from Fyne’s tiger-like appearance that he and the Doctor are spies for a figure named Seraph. It soon becomes apparent that the crew and the officers of the ship are at war, and Tx, the leader of the rebels, orders that the Doctor and Fyne be imprisoned along with Seraph’s daughter, Casimir.

Fyne immediately becomes infatuated with Casimir, a wild and beautiful half-woman, half-lion. She explains that she and her father found the people of Earth recovering from {devastating solar flares}, and helped to guide the survivors onto the three-fold path of peace, harmony and enlightenment. Some of the Earth’s inhabitants then turned against them, however, and Seraph and his loyal followers thus purchased this ship and set course for a new world. However, as they passed this black hole, Seraph became entranced by a signal which only he could hear, and piloted the ship into its current orbit. His mind has not since returned to his body, but the officers refuse to alter course, convinced that he will one day return. The crew have thus rebelled, believing that the officers have doomed them all.

Casimir claims to have visited the village to negotiate peace with Tx, only to be captured and held as a prisoner of war. Fyne, as an army officer himself, automatically assumes that the ship’s officers are in the right and the crew in the wrong. The Doctor advises him not to make facile assumptions, and warns him that his infatuation with Casimir may be a side-effect of his own transformation; she may have a hidden agenda. However, Fyne is thoroughly captivated by Casimir’s wild beauty and believes the Doctor to be mistaken.

Casimir admits that she knows of a way to escape from the village through the service tunnels, but this ship was old when they bought it, and some of the tunnels are occupied by dangerous creatures which have evolved to survive even in the vacuum of space. Nevertheless, the Doctor picks the lock into the service tunnels and reactivates one of the tunnel trains, hoping to reach the bridge and negotiate peace. Unfortunately, it turns out that the train was out of service for a reason, as the Doctor discovers when it crashes into another train blocking the tunnel. Casimir tries to proceed beyond the wreck, but is attacked by a tentacled monstrosity. While Fyne helps Casimir to fight the monster, the Doctor finds and reactivates a service robot trapped in the wreckage of the first train, and it helps him to drive off the creature.

The robot prepares to return to its pre-programmed duties, but the Doctor manages to convince it that the war may cause damage to the ship. The robot thus provides him with a map of the ship, including a secret entrance to the bridge. The Doctor intends to take the map to Tx and bargain with him, but Casimir strikes him down and takes the map; however, since she and Fyne both owe the Doctor their lives, she does not finish him off. Though his loyalties are now divided, Fyne still believes that the Doctor made the wrong choice and thus agrees to accompany Casimir to the bridge. This is just as Casimir had hoped, but she is troubled by the fact that she’s becoming attracted to Fyne as well.

The Doctor recovers and leads Tx and his men to the secret entrance, having memorised the map before Casimir took it from him. Casimir and Fyne make it through the entrance first, but the officers on the other side shoot Fyne with stun guns, and he awakens to find himself strapped down in a medical bay, next to Seraph’s unconscious body. Only Seraph can convince the warring factions to settle their differences, and since Casimir believes that Seraph is unlikely to return, she intends to transfer Fyne’s mind into Seraph’s body in order to trick Tx and his men into believing that Seraph has awoken and thus surrendering to the officers. Casimir admits, however, that the operation will be highly dangerous, and that Fyne’s honour and nobility have given her second thoughts.

Nevertheless, the operation is about to proceed when the Doctor breaks into the sickbay, having reprogrammed the ship’s service robots to fight alongside Tx and his men. The Doctor forces the medical robot to admit that, contrary to what Casimir wanted to believe, the procedure would inevitably have proven fatal to Fyne, although he would have lived long enough to serve the officers’ purposes. Casimir thus calls off the operation and allows the Doctor to use the medical equipment to reprogramme the nanoviruses within Fyne’s body. However, Fyne offers an alternative; instead of using his own human blood as a template to restore him to human form, he suggests that the Doctor use Seraph’s blood as a template instead. The procedure works, and Fyne becomes the physical image of Seraph, while retaining his own memories and personality. Casimir convinces Captain Sha and his officers to surrender to Tx, and once Fyne addresses the crew in the guise of Seraph, the war comes to an end.