Young Lucius is running away from Lavinia, who is following him and, because of her disfigurement, frightening him. Marcus and Titus tell him not to be afraid of Lavinia, who is attempting to communicate with gestures. Young Lucius thinks Lavinia may have gone mad, but Titus realizes she is trying to point out a book. Marcus interprets Lavinia’s raising up her two arms as meaning that two people hurt her.
Young Lucius’ fright at Lavinia’s disfigurement parallels the shock of the audience viewing the gory violence of Shakespeare’s play. In displaying on stage violence and disfigurement that is usually kept off-stage, Shakespeare forces the audience to directly experience the true cost of vengeance and violence.
Lavinia points to a copy of Ovid’s Metamorphoses and opens it to the story of Philomela. Titus realizes that Lavinia is saying she was raped, like Philomela, in the woods. Titus takes a staff and uses it to write in the dirt on the ground, guiding it with his mouth. He encourages Lavinia to do the same and write the names of those who have violated her. Lavinia writes “stuprum,” which is the Latin word for rape, and the names Chiron and Demetrius. Marcus, Titus, and Young Lucius swear to exact revenge on Tamora’s sons. Titus tells Young Lucius to bring some weapons to Chiron and Demetrius as presents.
Upon learning the identity of the culprits behind Lavinia’s rape, the Andronicus family immediately jumps to planning revenge, continuing the play’s cycle of violence. (Why it takes Titus and the others so long to realize that Lavinia might be able to communicate in this way isn't really explained and is probably better being ignored.)