Shain is based on the character of Shakushain, an Ainu chifetain who led an important rebellion (Shakushain no tatakai, 1669-1672) against the Matsumae clan, the Japanese lords who occupied the Hokkaidō region at that time.
The Noh follows a rather standard structure in which a monk (waki) visiting Hokkaidō meets a local man who tells him the story of Shakushain (shite) before disappearing. In the second half of the play the shite re-enters the stage in his real form, as the spirit of Shakushain. In the interview to the Nōgaku Times, Nagajima-sensei explains how the character for the nochi-ba (second act) was built around that of the Noh Kōu (on the Chinese general Hsiang Yu). One of the highlights of the play is the ezo-nishiki fabric costume for the character of Shakushain, of which you can find pictures on Nagajima-sensei’s website.
Unfortunately not much is said about the ‘post-colonial’ resonance of the play. Obviously the Japanese invasion and subjugation of Hokkaidō has an important meaning in Japanese history, especially if seen in the perspective of Tamura, the only classical Noh play dealing with a similar theme. In Tamura the general Sakanoue no Tamuramaro (758-811) is sent by Emperor Heizei to Mt. Suzaku to contrast an Ainu invasion, whom he defeats with the help of the bodhisattva Kannon, who mercifully sends a thousand arrows on the ‘demons of the North’, killing them all. You can find a synopsis on www.the-noh.com, and perhaps also notice how they artfully avoided to mention that Tamura’s enemies were actually Ainu people.
Those interested in attending Shain can find more info on Nagajima Tadashi’s website.