In mid-July of 1805, the expedition had still not sighted any Indians, although signs of their proximity proliferated. Soon, Sacajawea informed the expedition that they were in Shoshoni territory; she had noted and identified markings and abandoned camp sites. At this point, both Captain Clark and Charbonneau had fallen extremely ill. It was now crucial that the expedition increase their rate of travel if they were to reach the Pacific before winter; but this would not be possible without horses; an only the Shoshoni people could supply the animals.

Now in Shoshoni territory, the expedition came to the spot where Sacajawea remembered being captured by the Hidatsas at age twelve; on August 11, 1805, the expedition finally came into contact with its first Shoshoni, a warrior wandering alone. Lewis made a gesture of peace by laying a blanket on the ground, and the warrior approached. However, threatening movements by Lewis' subordinates scared him off. This did not bode well for establishing good relations with the Shoshoni, or for acquiring horses. Luckily, the expedition had Sacajawea with them.

On August 13, the party met a group of Shoshoni women who, because they had children with them, were unable to run away quickly. They expected to be killed, but Lewis and Clark surprised them with presents instead. The women then led the expedition to the Shoshoni camp, where they exchanged more gifts and smoked pipes. (This was before Clark and Sacajawea made it to the camp; the captains had divided the expedition members between them and had been patrolling separately; Sacajawea had been in Clark's group.) Most of the presents Lewis and initially gave to the Shoshoni were Jefferson medallions, depicting the face of this man the Indians called the "Great White Chief." The Shoshoni chief, Cameahwait, treated Lewis and his men with kindness, though most of the other Indians feared the white men.

When's Clark's group made it to the Shoshoni camp, Sacajawea was overjoyed. After all, she had been separated from her people since early girlhood. Clark noted that she danced happily when she saw that a Shoshoni party was approaching them. Sacajawea proved her value as an interpreter, and surprised everyone when she and Cameahwait determined they were brother and sister.

Initially, Cameahwait haggled and traded only a handful of horses to the expedition. Lewis and Clark were very happy to have three or four. Later, however, Cameahwait would relent and give his sister's friends many more. When it came time to leave the Shoshoni, Sacajawea did not stay, but loyally followed her husband and the Lewis and Clark expedition.

One of the major reasons the Lewis and Clark Expedition went so long without meeting any Shoshoni was that Sacajawea accompanied them. If this woman carrying a baby had not been present, the Shoshoni might have perceived the white men as a threat and attacked them before they could come too close to their village. But Sacajawea and her child marked the expedition as peaceful. Moreover, the Shoshoni of this region were themselves a peace-loving people. Instead of guns, they still used bows and arrows.

When the expedition came across the place where the Hidatsas had captured Sacajawea, Lewis noted in his journal that Sacajawea showed a surprising lack of emotion. He inferred from this that she was incapable of deep emotion because of "savage simplicity." However, when she reached the actual Shoshoni camp and was reunited with her brother Cameahwait, Sacajawea proved Lewis wrong by showing a great deal of sentiment. She was overjoyed to see many of her old girlhood friends. Her happiness was mingled with tears, however, as she learned from Cameahwait that almost everyone else in her family had died since her capture. Even her sister had died, leaving behind a small boy. Among Sacajawea's old acquaintances still alive was a Shoshoni man who claimed that he and Sacajawea had been betrothed since her childhood, but when Sacajawea explained that she was already married to Charbonneau he dropped his claim.

The Shoshoni were facing a difficult part of the year, when food was scarce. They did not use guns, and hunting with bows and arrows produced little big game to eat. Lewis's men quickly made friends with the Shoshoni by going out and killing several deer with their guns. The Shoshoni devoured one of these deer raw, right where it had been shot.

Sacajawea helped the Lewis and Clark expedition negotiate the transaction for horses with the Shoshoni. Because she was related to Cameahwait, she ended up being more valuable to the expedition than Lewis and Clark had ever expected. Those who are critical of U.S. policy towards Native Americans in the subsequent years may also be critical of Sacajawea, helping the Lewis and Clark expedition open the western frontier for exploitation by white settlers. Was she a hero for helping Lewis and Clark's mission of exploration? Or was she a traitor for cooperating with a system that would ultimately destroy the Shoshoni way of life?