A Federation transport carrying some two thousand colonists which suffers a hull breach in 2365, forcing the Enterprise-D to leave an away team at Gravesworld and race to its aid.
The USS Constantinople is mentioned in the Next Generation episode The Schizoid Man. The Enterprise is on its way to Gravesworld to give medical help to the dying scientist Ira Graves when the transport reports a hull breach that threatens the roughly two thousand colonists on board.
Picard cannot do both jobs at once, so he drops an away team of Data, Troi, Worf and Doctor Selar using a risky near-warp transport and carries on to the Constantinople. The rescue takes place off screen and is reported as a success.
The detour matters to the plot because it leaves Data alone with Graves for long enough for the scientist to copy his own mind into the android's positronic brain before the Enterprise comes back.