The crew loses a day, and Data won't say why.
The Enterprise passes through what seems to be a wormhole, and the whole crew except Data loses consciousness for about thirty seconds, or so Data says. Small inconsistencies begin to pile up: Dr. Crusher's cell culture has grown more than it should, and Worf has a healed wrist injury he did not have before.
Data lies to Picard and refuses to explain, raising fears that he has been compromised. Eventually it is revealed that the crew lost about a day after meeting the Paxans, a reclusive species that values its privacy and wanted to destroy the ship.
Picard had ordered Data to keep the encounter secret as part of a deal with the Paxans. With better precautions, the crew agrees to repeat the encounter so that the secret holds this time.