The Sheriff of Eternity and Jack are waiting for rustlers. They have men hidden away for an ambush. Suddenly, the hidden men are attacked by something with "teeth like knives, skin like a tree trunk and glinting, alligator eyes". The Sheriff and Jack are soon killed as well.
The Doctor takes Peri to the Wild West in the 1880s to see Peri's past. They enter the saloon, run by Josh Larson, and introduce themselves. Josh thinks that the Doctor is Dr Sebastian Rowley, whom they are expecting to arrive to investigate the recent strange deaths. The Doctor goes along with the assumption. The deputy Sheriff, Paul, comes in with a plaster footprint. The Doctor recognises it as a dinosaur called a deinonychus.
Peri, meanwhile, has gone for a walk with Josh, who, despite looking like a man in his twenties, claims to be one hundred and ten years old. He tells Peri of an elixir he bought when he was eighty. When he took it, he started becoming younger, and now only ages about a year for every ten. When Peri tells him she has a relative named Iziah Brown, he takes her to see the Brown family.
Paul tells the new Sheriff about the arrival of "Dr Rowley". The Sheriff asks for a description and recognises both the Doctor and Peri.
The Doctor is in a shop looking at fossils, and meets Sarah Brown, wife of Iziah Brown. She knows of Dr Rowley and is thrilled to finally meet him. She tells him of dinosaur bones found on her land and offers to take the Doctor there. However, the Sheriff enters, and the Doctor discovers he is the Master. The Doctor is locked up. When Paul tells the Master about Peri, the Master decides killing her will make the Doctor suffer, and he and Paul leave. Sarah shows up with explosives and frees the Doctor. However, they are confronted by the Master and hypnotised townspeople. The Master kills Sarah when he learns her last name, and gloats that he has just wiped Peri from history. He then tells the people that the Doctor is an imposter and has just killed Sarah Brown. The townspeople begin to attack the Doctor.
Peri has met Iziah Brown and is surprised to learn that he has six wives. She wonders which of them might be her ancestor.
As the Master gloats over the Doctor, the Doctor pulls at the mask that is covering the wounds the Master sustained on Sarn. The Master tells the Doctor he has found the Fountain of Youth, orders the townspeople to kill the Doctor, and leaves.
Iziah tells Josh and Peri his family's secret: they have found the Fountain of Youth.
Meanwhile, the townspeople have strung up the Doctor, and after fifteen minutes cut him down.
Iziah and his five wives take Josh and Peri to the cave where the fountain is located. The fountain is in an area littered with dinosaur fossils. As they approach it, the Master appears with half a dozen armed, hypnotised men. He herds Iziah's group away from the fountain and fills a bucket with the water. Before he can use it, Peri slams into him, spilling the water. All around them, the dinosaur fossils that the water touched turn into living dinosaurs...
As more velociraptors regain life, the Master orders his gunmen to shoot them. During the ensuing carnage, Peri begins to herd Josh and the Browns to a nearby exit. As they make their way out, the Master notices and grabs Dana. Peri and Josh go back to rescue her, but the Master strangles her, hoping to erase Peri. Peri, enraged, throws Josh's lantern at the Master, and he begins to burn. Peri and Josh join the Browns and make their way back to the Brown house.
The Master is attacked by a velociraptor but manages to shrink it with his Tissue Compression Eliminator. The shrunken raptor falls into the fountain and regains its life. The Master, seeing this, drenches himself in the fountain water and is healed. He proceeds to hypnotise the dinosaurs.
The Doctor has been cut down and is being examined by the undertaker, Curtis, and his assistant Pete. They argue about whether or not to bury the Doctor. Curtis sends Pete to get a sack for the body, and is surprised when the Doctor sits up and speaks. Curtis runs off, and the Doctor heads back to the saloon, empty except for a blind man who tells him the townspeople are headed for the Browns' home. The Doctor obtains a horse and heads there himself.
At home, the Browns gather weapons and dynamite and prepare for a dinosaur attack. They are instead attacked by the hypnotised townspeople, led by Deputy Paul. Armed with sticks and tools, they attack Peri. The Doctor appears and rescues her, bringing her inside the house.
The Doctor has an idea for dealing with the angry mob without hurting them. They will shock the townspeople out of their hypnotism. When the mob finally breaks through the door, they are confronted by the Doctor, Peri, Josh and the remaining Browns engaging in odd behaviour - singing, dancing, hugging, juggling, and barking. With the hypnotism broken, Josh orders them back home, and they comply.
However, the Master has shown up with his hypnotised dinosaurs. Before he can order them to attack, the Doctor shoots him. The men rope the dinosaurs, and when the Master recovers from the gunshot, Josh loops one of the ropes around the Master, and sends the dinosaur connected to it on its way, with the Master dragging behind.
Later, the Doctor goes to the cavern and sets up dynamite for the fountain. He buries the Fountain of Youth until some future time.
- Fifth Doctor
- Peri Brown
- The Master
- Josh Larson
- Iziah Brown
- Sarah Brown
- Simone Brown
- Dana Brown
- Millicent Brown
- Venus Brown
- Wanda Brown
- The Master uses the alias Sheriff Masters.
- The Master recognises the velociraptors from Jurassic Park.
- The Master uses his Tissue Compression Eliminator.
- The Doctor's respiratory bypass system allows him to survive the hanging.
- This story is split into two parts.