The Chûshingura theatrical tale was based on on actual events from 1703 when former retainers of the lord of the Akô domain, Asano Naganori, exacted revenge by murdering Lord Kira Yoshinaka, who had (apparently) so enraged their lord that Asano attempted to mruder Yoshinaka. Asano's action was an serious violation of the samurai code of behavior within a shogunal palace, whose punishment resulted in Asano's seppuku (lit., "incision of the abdomen," ritual suicide: 切腹).
The oldest surviving Chûshingura play is Goban Taiheiki (Chronicle of great piece played on a chessboard) written in 1706 by Japan's great playwright Chikamatsu Monzaemon. The plot involves the historical Kô no Moronao (高師直 died 1351), who was the first to hold the position of Shitsuji (Shogun's deputy) and became general of the Shogun's (Ashikaga) armies, which defeated the forces of the southern court in the fourteenth century. However, the genesis of Chikamatsu's story can be found in a puppet play also by him written less than a month earlier called Kenkô hôshi monomigurugusa (The sightseeing carriage of the priest Kenkô), in which the priest persuades a general named Kô no Moronao to transfer his unwanted libidinous attentions from a court lady to the wife of Enya Hangan. When she rejects Moronao, he denounces her husband and forces him to commit seppuku. Thus the catalyst for future theatrical treatments and their various expositions of the vendetta had been set by two Chikamatsu plays in 1706. Also established was the transfer to the sekai (world or sphere: 世界) of the fourteenth century. Naturally, this sekai resonated with another rousing saga, the Taiheiki (Chronicle of great peace:太平記), a historical epic from that era covering the period 1319-67. It deals primarily with the Nanboku-chô (1336-92), a period of war between the Northern Court of Ashikaga Takauji in Kyoto and the Southern Court of Emperor Go-Daigo in Yoshino.
The play depicted in Shigeharu's triptych, Taiheiki chûshin kôshaku (Story of the loyal chronicle of great peace: 太平記忠臣講釈), premiered in Edo at the Ichimura-za in the second month of 1766. It was an adaptation of the foremost puppet and kabuki version, the 1748 Kanadehon chûshingura (Copybook of the Treasury of Loyal Retainers: 假名手本忠臣蔵, often called simply "The Forty-seven Rônin"), whose dramaturgical re-imagining features a vendetta by the retainers of Enya Hangan (a provincial daimyô) who committed seppuku after a confrontation incited by Kô no Moronao (a chief councilor to the Shogun).
The doubling structure of the Taiheki epic/plays with the Chûshingura dramas revolve around refashionings of the actual Kô no Moronao. In the historical Taiheiki, he is portrayed as a villain who is accused of unbridled violence, greed, and lewdness. It is this earlier, long-standing reputation that must have appealed to the Chûshingura playwrights when they sought a villain for their revenge tale to be set in a distant sekai, as they could not name the real-life figures in the Asano affair for fear of running afoul of the shogunate's censorship edicts.
Taiheiki chûshin kôshaku seems to have been the Naka Theater's rival production to the Kado's staging of Taiheiki samurai kagami (Mirror of the Taiheiki warriors: 太平記土鑑) in 10/1835, which featured Nakamura Shikan II. In the scene shown here, Wakasanosuke is a young lord (daimyô: 大名) who, along with others, is treated shabbily by Moronao. When Moronao commands both Hangan and Wakasanosuke to appear at the shogun's palace in order to prepare the ceremonies for the Shogun's younger brother (whom they are to entertain), Wakasanosuke is intent on using his entry into the palace as an opportunity to settle the score. Yet it is Hangan who actually strikes Moronao for his insults, and is then restrained from killing him. This leads to Hangan's seppuku and the subsequent vendetta by his loyal band of [now] masterless samurai (rônin: 浪人).
References:WAS-IV, no. 552; KNP-6, p. 302; IKB-I, no. 1-496 and 1-498; OSP, no. 170