The play is a moral allegory about Mankind, a representative of the human race, and follows his fall into sin and his repentance. The audience is instructed in the proper Christian life by watching Mankind's fall and redemption.
The play begins with Mercy, who instructs the audience in how they should behave but is soon interrupted by Mischief (whose name indicates, in fifteenth-century English, something much more serious than the 'prank' it means now). Mischief mocks Mercy's preaching. A page is then missing in the manuscript. When the play resumes, Mischief has departed and New Guise (i.e., 'Fashion), Nowadays (i.e. 'living for today') and Nought (i.e. 'nothingness') are on stage. They continue the mockery of Mercy. After their exit, Mercy again addresses the audience, explaining why the three are evil and urging the audience to not follow their example.
Mankind enters, and addresses the audience, introducing himself. He is a farmer, and is resolved to live a virtuous life. Mercy instructs Mankind about how to continue in this goal, warning him specifically about Mischief, New Guise, Nowadays and Nought. But Mankind has to stand against temptation on his own, and so Mercy leaves.
New Guise, Nowadays and Nought return to tempt Mankind. First, they encourage the audience to join in with a scatological song. Then they turn their attention to Mankind, but he successfully resists their enticements and beats them off with his spade.
Mischief returns and conspires with New Guise, Nowadays and Nought to bring in a greater devil, Titivillus. But first, they demand that the audience pay money before they can see Titivillus. When the audience does so, Titivillus enters, and begins immediately making Mankind's life difficult. The audience can see him, but he is invisible to Mankind. He hardens the ground, making it hard to farm. He steals Mankind's seed and spade. He induces a need to urinate. He distracts him from his prayers. Finally Mankind becomes so frustrated that he gives up and goes to sleep, and Titivillus whispers to him that Mercy is dead. Through his scene, Titivillus implores the audience to keep silent and watch him, which makes the audience complicit in his actions.
Thus deluded, Mankind rejects Mercy and goes to join New Guise, Nowadays and Nought. But now their full evil is revealed. Mischief has been in jail, and has escaped, robbed the jailer, and raped his wife. New Guise has narrowly escaped hanging. Nowadays has robbed a church. The three have Mankind swear vows to join their gang, vows which also show their depravity—to be a highway robber, to seduce women, to 'rob, steal, and kill as fast as you may go' (708). When they leave, Mercy returns to plead with the audience about the unreliability of Mankind, and to pray for his redemption.
The three reveal to Mankind that Mercy is not really dead, and tempt him to kill himself rather than face Mercy. But Mercy chases them away.
The final struggle for Mankind's redemption is with himself. Mercy tells him all that he must do to be forgiven is to ask, but Mankind finds this difficult and raises a series of objections. The theology of the ending focuses on perseverance, on living as a Christian even while continuing to sin, even while trying not to, and having to repeatedly ask for mercy for those offences. Mankind finally accepts Mercy, and then promptly tries to blame the devils for his problems, but Mercy reminds him that he warned Mankind about them. For the play ends with Mercy addressing the audience again, exhorting them to repentance.
'Mankind' is the subject of an opera, written for St. Peter's school in Solihull, by Michael Finnissy and first premiered in Olton Friary in 2008 with 300 students from St. Peter's school and four Primary feeder school, directed by Dr. Stephen Lansberry.