This is a jidai-mono (historical plays) gidayu-kyogen, based on the incident when Sugawara no Michizane (also known as Kan Shojo), a statesman and member of the nobility in the Heian period (8th to 12th century), was demoted and sent away due to a plot by Fujiwara no Shihei.
Specific scenes of this play are usually performed today, such as ‘Domyoji’ (The Domyoji Temple) scene that shows a miracle occurring when Kan Shojo, who is to be exiled, parts from his aunt and daughter, ‘Kurumabiki’ (The Fight Over the Carriage) scene in which the triplets Umeomaru, Sakuramaru, and Matsuomaru split into friend and foe and fight, ‘Ga no Iwai’ (The Birthday Celebration) scene in which Sakuramaru commits seppuku (ritual suicide) as he regrets creating the catalyst for Kan Shojo’s exile, and ‘Terakoya’ (The Village School) scene in which Matsuomaru sacrifices his own child to save Kan Shojo’s child, Kan Shusai.
“Sugawara Denju Tenarai Kagami” is considered one of the three great masterpieces of gidayu-kyogen, along with “Kanadehon Chushingura” (The Treasury of 47 Loyal Retainers) and “Yoshitsune Senbon Zakura” (Yoshitsune and the Thousand Cherry Trees).
Takebe Genzo, a master of writing, used to work for Kan Shojo. In ‘Terakoya’ scene, there is a scene in which Genzo presents the head of a child who has just enrolled in the school for inspection as a substitute for the head of Kan Shusai. The inspection is carefully carried out to confirm that the head presented is the real thing. The tension-filled conversation between the two characters is one of the highlights of this play. However, later on the audience sees an unexpected development: it is revealed that the child whose head was substituted was the son of the inspector, Matsuomaru.
Scenes like this one, along with instances of visual confirmation - inspections to check whether a severed head is that of the correct person - are called kubi jikken. There are many other gidayu-kyogen in which the key to story development is a substituted head in a kubi jikken scene.