Iago and Roderigo are waiting for Cassio. They have put their plan into motion. Roderigo is going to kill Cassio. Iago has given Roderigo a sword, so that he can kill Cassio.
Cassio then emerges. Roderigo stabs at Cassio but fails to pierce Cassio's armor. Cassio stabs and wounds Roderigo. Iago darts out in the commotion, stabs Cassio in the leg, and exits. Not knowing who has stabbed him, Cassio falls.
Othello then comes by. He hears Cassio's and Roderigo's cries. He believes that Cassio's cries are because of the plan that he and Iago had planned earlier.
'Oh, Cassio cries, bloody murder. This must be the work of Iago. I praise him for doing such an excellent job.'
Lodovico and Graziano also hear the cries from Cassio and Roderigo. They try to help, but cannot because it is dark out. They also believe that cries might be a trap. Iago then enters carrying a light. First he pretends to discover Cassio, who begs him for help.
"You must help me! This is murder! MURDER I CRY!"
Iago just stands there, looking a t Cassio. He then comes upon Roderigo. Roderigo cries for help.
"Help! Someone help me!"
Cassio then says that Roderigo is one of his attackers.
"You, murderer! You Villain!" cried Iago. Iago then stabs Roderigo. Roderigo is now dead.
Bianca enters and begins to cry out when she sees the wounded Cassio. Iago questions Cassio about his assailant, but Cassio can provide no explanation for what happened. Iago suggests that Roderigo is to blame. Cassio says that he does not know Roderigo. Attendants carry off Cassio and Roderigo's corpse.
Emilia then enters, and Iago tells her what has happened. He and Emilia punish Bianca, at whose house Cassio had dined that evening. Iago takes Bianca under arrest, and sends Emilia to tell Othello and Desdemona what has happened.