Everyone goes out onto the battlefield in a blaze of glory. Young Cato runs around shouting his name as a challenge to anyone who stands for tyranny and against the Roman Republic.
Lucilius is running around pretending to be Brutus.
Some enemy soldiers unceremoniously kill Young Cato. They're ready to kill Lucilius too, but he says he's Brutus, and they should be honored to kill him.
The soldiers take him prisoner and are excited to show off their catch to Antony. (They really believe he's Brutus.)
The captive Lucilius tells Antony that no one will ever take Brutus alive. Lucilius promises that when Antony finds Brutus, whether alive or dead, he'll still be Brutus, with the same noble character and unchanged by these events.
Antony tells his overeager soldiers that this guy isn't Brutus, but he's no less worth capturing. Antony orders the soldiers to keep Lucilius safe and to be kind to him, as he'd rather have such men for friends than enemies.
Antony then sends some folks off to find out whether Brutus is alive or dead. He goes to Octavius's tent to hear news of how things are going.