Shasta was born in 1000 NT as Prince Cor, the eldest of King Lune's twin sons. Shortly after birth he was abducted by a corrupt Archenlander after it was prophesied that one day Cor would save Archenland from it's greatest danger. But the ship carrying the traitors and baby was attacked and a knight escaped with Cor in a small boat. It was many days until they found land and the knight died of starvation and thirst, choosing to protect Cor instead. There, the boat drifted into the Calormen coast and Cor was found by a poor fisherman named Arsheesh. The man named the baby Shasta, and raised him as his own son, eventually treating him no better than a servant.
One day when Shasta was 14, he overheard a conversation between the fisherman and a Tarkaan lord who wanted to buy Shasta as a slave. Through that Shasta learned that he wasn't Arsheesh's son at all, and this knowledge filled him with sadness and joy at the same time.
He was in the stable talking over the situation to himself when the Tarkaan's horse spoke up. Shasta discovered that the horse was named Bree, who was really a talking horse of Narnia. Bree said that Shasta would rather be dead than be a servant in the Tarkaan's care, and proposed that they both escape to Narnia. Shasta agreed, and they set off that night.
As they went, they were pursued by a lion, and because of this, crossed paths with a horseman who at first appeared to be another Tarkaan. But the rider was in fact a girl, named Aravis, who was running away from an engagement with a decrepit old Tarkaan, and what was more, her mount, Hwin, was also a Talking Horse of Narnia.
The four joined forces, somewhat unwillingly, and continued to make their way North. They had to pass through the great city of Tashbaan, and attempted to do so quietly. However, all such hope of this was irrevocably dashed when Shasta was mistaken for Prince Cor of Archenland by a band of Narnians and was whisked away.
Shasta was taken to the Narnianís apartments, where his true identity was still not uncovered. He was made to rest on a couch, and there overheard all their plans for a secret escape from Tashbaan to Narnia. Shasta became afraid that if the Narnians knew he was not really Prince Corin, as they believed him to be, they would kill him for knowing their plans.
As he was trying to figure out how to escape, a boy climbed over the windowsill. It was Prince Corin, and indeed, he looked exactly like Shasta. They became friends very quickly, and Corin gave Shasta instructions on how to get out of the city safely. He also gave instruction to seek him out at his home in Anvard, and told him the name of his father, King Lune. Shasta then departed, and went to the Tombs of the Kings, on the outer walls of Tashbaan.
He waited there a terribly long time for Aravis and the horses. Shasta had to spend a frightening night near the Tombs, which seemed an especially spooky place. A lion came and frightened off the jackals, and the appearance of another lion frightened Shasta. But a friendly house cat came to him and comforted him with its presence.
When Aravis and the Horses finally arrived, Shasta told them about the Narnians, and Aravis shared some information she had gathered during her time in Tashbaan; the gist of it being that Rabadash, prince of Calormen, was planning to attack Archenland as soon as he could gather men and horses. Shasta, Aravis, Bree and Hwin immediately embarked across the desert to Archenland, traveling fast so as to get to Anvard before Rabadash. Using the instructions that Shasta had gained while staying with the Narnians, the successfully crossed the desert, and made it into the outer reaches of Archenland.
Once in Archenland, the travelers were pursued by yet another lion, who chased them towards the home of a hermit, and also injured Aravis. Bree would not stop running to help Aravis, so Shasta jumped off himself and shouted at the lion, who then ran away.
Upon receiving shelter at the Hermit's home, Shasta was immediately commanded to run straight ahead of him, until he came to King Lune, so that he could warn him about the impending attack. Shasta did so, and found the King in time. After telling them about Rabadash, Shasta was given a horse and told to follow them to Anvard. However, Shasta was not accustomed to riding a non-Talking beast, and soon fell behind and became lost.
While lost and alone, Shasta had an encounter with an unseen being who traveled beside him and spoke to him, listening to Shasta's troubles and correcting and explaining things to him. It was soon revealed that the being was Aslan, who had taken the form of other lions and creatures, and had been following and guiding Shasta the whole time. When day broke, Shasta was finally able to see Aslan next to him, and he fell on his knees. Aslan blessed him with a kiss, and then disappeared.
Shasta encountered some Dwarfs, who gave him food and let him rest. There, he reunited with the band of Narnians from Calormen, who were on their way to help King Lune. Prince Corin was there, and although expressly commanded not to fight in the battle, he snuck in anyway, and dragged Shasta along with him. Shasta rode into battle as bravely as he could, but was quickly knocked off his horse and got tangled in the fray. Fortunately, he suffered only scraped knuckles.
When the fight was over, Shasta was presented to King Lune and it was revealed that he was the king's long-lost son, and Prince Cor's older twin brother, making him first in line for the throne of Archenland.
Prince Cor, as he was now known as, grew up in Anvard along with his reunited family and Aravis, who had been invited to stay as a lady of the court. Cor and Aravis eventually married and were good rulers of Archenland, and Cor was counted among the faithful at the Great Reunion.