Before the Battle of Changping, the state of Zhao was no weaker than the state of Qin. Qin attacked Zhao twice, but both times were repelled by the Zhao general Lian Po. To negotiate peace, Qin sent Yiren, the son of Lord Anguo (the crown prince of Qin), to Zhao as a hostage. Yiren was a middle-ranking son of Lord Anguo, born of a concubine, and his mother, Xia Ji, was not favored. After Yiren arrived in Zhao, Qin continued to frequently attack Zhao, so the people of Zhao treated him poorly, leaving him in a difficult position. Yiren lived in Zhao like a prisoner, struggling in poverty, with scarce resources for carriages and money, and felt that his entire life would be wasted in Zhao.
Lü Buwei was originally from the State of Wei and made his living as a merchant. Traveling from place to place, buying cheap and selling dear, he amassed a vast fortune. Once his business grew large, the small State of Wei could no longer satisfy his commercial ambitions, so he moved to the State of Zhao. Zhao was a key transportation hub in the Central Plains at the time, with a developed economy that favored trade and commerce. While doing business in Handan, Lü Buwei happened to meet Yiren, and with the shrewd instincts of a merchant, he realized that this man was "like a rare commodity that could be stockpiled, waiting for the right moment to sell," and so he sought every means to befriend him.
Lü Buwei first bribed the guards watching over Yiren, allowing himself to visit Yiren. He said to the disheartened Yiren, "I can make your family shine." Yiren did not believe him and said with a bitter smile, "First make your own family shine, then come and make mine shine." Lü Buwei replied, "You don't understand—my family's glory depends on yours." Alone in a foreign land with no trusted confidants, Yiren soon came to regard Lü Buwei as indispensable.
Yiren understood exactly what Lü Buwei was implying. Having spent his days in Zhao with no clear direction, he asked Lü for advice. Lü said, "The King of Qin is old, and the Crown Prince is your father, Lord Anguo. I have heard that his most favored consort is Lady Huayang, but she has no children yet. Given her level of favor, she can absolutely influence Lord Anguo's decisions. You have over twenty brothers, and you rank neither first nor last, with no particular strengths, no favor, and you have been in Zhao for a long time. Even if the King of Qin passes away and Lord Anguo becomes king, you would have no leverage to compete with your elder brothers and other brothers for the position of Crown Prince." Yiren said, "Yes, you are right. But what can I do?"
Lü Buwei said, "You have little wealth and live under someone else's roof, so you cannot afford to offer gifts to please your relatives and friends or build up your influence. Although I am not particularly wealthy myself, I am still willing to provide you with funds and make a trip to the State of Qin on your behalf to meet with Lord Anguo and Lady Huayang, and persuade them to make you the crown prince."
Having said this, Lü Buwei indeed gave five hundred gold pieces to Yiren, allowing him to replace his carriages and horses and widely befriend retainers. At the same time, he himself took another five hundred gold pieces, bought some rare treasures, and traveled west to Xianyang to open up connections. He first visited Yangquan Jun, the brother of Lady Huayang, and her elder sister, getting Lady Huayang's sister to personally go and act as a persuader to Lady Huayang herself.
The elder sister advised the younger: "In youth, you can rely on beauty to gain favor, but one day your looks will fade, and you have no children to depend on—what will you do then? It's better to pick a capable and filial son among the crown prince's sons early on, adopt him as your own, and make him the heir to the throne. That way, while your husband lives, you can enjoy status through him; once he's gone, the heir will be your son, and you can gain honor through him—no need to fear losing power. Listen to me: if you don't build your foundation now while you're favored, once you fall out of favor, will the crown prince still listen to you? I have a candidate: Yiren, who's been sent as a hostage to Zhao. This child has good character and knows he can't be heir due to his birth order, and his birth mother is also unloved. If you support him as heir, he'll be eternally grateful and treat you with lifelong filial piety—then won't your lifelong honor and respect be secured?"
These words struck a chord with Lady Huayang, who deeply agreed with them. Lü Buwei seized the opportunity to present the rare treasures he had brought to Lady Huayang, while praising Yiren's intelligence and virtue, saying that his followers were numerous across the land, and added, "Our Yiren regards you as heaven itself; because he misses you and the Crown Prince, he cannot sleep at night and weeps all day long."
Lady Huayang was delighted upon hearing this, so she seized an opportunity to speak with Lord Anguo, telling him that Yiren, who was a hostage in the state of Zhao, was highly talented and praised by many. She then tearfully said, "I am fortunate to have your favor, but I have no son, which troubles me deeply. I hope you will allow Yiren to become my son and be named your heir, so that I may have someone to rely on in the future." Lord Anguo agreed and designated Yiren as his successor. After the succession was settled, Lord Anguo and Lady Huayang sent many gifts to Yiren and appointed Lü Buwei as his tutor, causing Yiren's reputation to grow ever greater.
One day, Yiren was drinking with Lü Buwei when Lü's concubine Zhao Ji came forward to perform a dance. Yiren took a liking to her and asked Lü to give her to him. Zhao Ji then became Yiren's wife and soon gave birth to a son, whom they named Zhao Zheng. Shortly after, the Qin state attacked Zhao again, besieging the Zhao capital of Handan. The situation grew dire, and the people of Zhao wanted to kill Yiren to vent their anger. Panicked, Yiren hurriedly consulted Lü Buwei. They bribed the city guards heavily and fled in haste to the Qin army camp. In their rush, they left Zhao Ji and their son Zhao Zheng behind in Zhao. Unable to capture Yiren, the Zhao people sought to kill his wife and child. Zhao Ji hid in the home of a wealthy Zhao household, pretending to be their daughter, and thus escaped the danger.
After Yiren returned to the State of Qin, Lü Buwei had him wear Chu-style clothing to meet Lady Huayang. Lady Huayang, who was originally from Chu, was especially delighted to see Yiren dressed this way, so she renamed him Zichu. Soon after, the King of Qin passed away, and Lord Anguo succeeded the throne, but within just a few days, Lord Anguo suddenly died from poisoning. In 251 BCE, Yiren became the new ruler of Qin, honoring Lady Huayang as the Queen Dowager, appointing Zhao Zheng as the crown prince, and Lü Buwei as the prime minister, granting him a fief of ten thousand households.