Short answers, pulled from the story.
The Xi'an Incident occurred on the 12th of December 1936 when Zhang Xueliang arrested Chiang Kai-shek to force a united front against Japan. This political crisis involved Nationalist generals seizing the leader to end civil war and resist Japanese invasion.
Zhang Xueliang and Yang Hucheng led the coup against Chiang Kai-shek while Zhou Enlai negotiated the terms with the Communist Party. William Donald, T.V. Soong, and Soong Mei-ling facilitated negotiations that secured Chiang's release.
Stalin viewed Chiang as indispensable because his removal might allow pro-Japanese figures like Wang Jingwei to take control of China. The Soviet Union feared that without Chiang, they would face extreme danger from a potential Japanese invasion.
Zhang Xueliang remained under house arrest for over 50 years until 1990 following the deaths of both Chiang Kai-shek and Chiang Ching-kuo. He was sentenced to ten years in prison by court-martial on the 31st of December before Chiang commuted the sentence.
Chiang Kai-shek verbally agreed to halt civil war, resist Japan, reorganize the Kuomintang, and approve an alliance with Russia and cooperation with the Communist party. These terms were finalized through negotiations involving Zhou Enlai, T.V. Soong, and Soong Mei-ling.