The "great captain" Sin Ji-Hun was a legendary 16th century Chinese seafarer. He was kept alive for hundreds of years by Sea Devil technology, but freed by the Thirteenth Doctor and her companions in 1807, sacrificing his life a short time afterwards to help thwart the Sea Devils' plan to flood the entire Earth.
The life of a great captain
In or slightly before 1533, Ji-Hun found the lost treasure of the Flor de la Mar. It was onboard his ship when he was contacted by a zealot Chief Sea Devil who believed that the Keystone, a powerful gem itself of Sea Devil origin, was part of the treasure. The Chief Sea Devil also wished to acquire a human vessel so that he could use it to "strike fear" into the hearts of the land-crawler, with him and his followers becoming pirates in their own right.
Ji-Hun pretended to swear allegiance to the Chief Sea Devil, "forcing" his crew off the ship. However, realising the significance of the Keystone, he had actually removed it from the bulk of the treasure and entrusted it to his second-in-command Lei Bao; furthermore, he was only going along with Chief Sea Devil's plan so as to ensure the human crew would be disposed of non-lethally. Immediately after he finished emptying the ship of human crew, the Chief Sea Devil prepared to kill him, reneging on their sham of an agreement. Ji-Hun began fighting back. Eventually realising that the Keystone was missing and Ji-Hun had tricked him, the Chief Sea Devil imprisoned Ji-Hun in his base, keeping him alive in stasis for hundreds of years as a form of punishment, meant to serve as an example for others who would seek to "resist his will". (TV: Legend of the Sea Devils)
Return to the world
The Chief Sea Devil was kept frozen in stolen by Bao and his descendants until he was released in 1807, which also awoke the rest of the Sea Devil crew. Ji-Hun was also freed when the Chief Sea Devil showed off his prisoner to the Thirteenth Doctor and Yasmin Khan, whom he had captured. He explained the nature of the Keystone to them, whereupon the Doctor managed to hit the necessary controls to abruptly make Ji-Hun's ship rise from the depths.
In the chaos, the two time-travellers freed Ji-Hun and went back to Madam Ching's ship with him. However, it transpired that Ying Ki, also aboard that same ship, was carrying the Keystone on him, being its sworn custodian as Lei Bao's last descendant; realising this, the Chief Sea Devil teleported in and grabbed it before returning to his own ship. The Doctor, Yaz, Ching, Ying Ki, Dan Lewis and Ji-Hun followed him there and in the ensuing swordfight, Ji-Hun gained the upper hand over the Chief Sea Devil, stabbing him to death; as he made clear to the Doctor, he had never intended to show mercy to the creature that had taken "[his] ship, [his] crew and [his] life" from him.
However, before dying, the Chief Sea Devil had transferred the Keystone to his crew's underwater base, where the machinery meant to cause all land on Earth to become flooded was already in motion. Ji-Hun, Dan, the Doctor and Yaz hurried back to the Sea Devil base, where he directed the travellers to recover what was left of the treasure of the Flor de la Mar from his ship. After the Doctor rigged up a system to make the base explode which needed someone to stay there until the explosion to hold it in place, intending to sacrifice her own life, Ji-Hun insisted on taking her place, explaining that he felt he had no place in the modern world outside, but wished to do what he could to salvage it for the people of this era. (TV: Legend of the Sea Devils)