Read it through once

Act fourth displayed the despairing Roderigo on the point of stabbing himself, because he has been told that Zara has deserted him. Just as the dagger is at his heart, a lovely song is sung under his window, informing him that Zara is true, but in danger, and he can save her, if he will. A key is thrown in, which unlocks the door, and in a spasm of rapture he tears off his chains, and rushes away to find and rescue his lady-love.