Picard: Now hear this. Printout message, urgent, all stations on all decks: prepare for emergency saucer-sep. You will command the saucer section, Lieutenant. Worf: I'm a Klingon, sir. For me to seek escape when my Captain goes into battle. Picard: You are a Starfleet officer, Lieutenant. Worf: Aye, sir.

Picard is going to seperate the saucer from the rest of the ship and needs Worf to command it. Worf has a difficult time between his Klingon nature and his Starfleet role.