S5E18 · Cause and Effect

The Enterprise is caught in a time loop (referred to in-universe as a "temporal causality loop"). It begins with the senior officers playing poker and continues for about a day until they encounter a spatial anomaly. While studying the anomaly, a Federation ship emerges from it, on a collision course with the Enterprise. Commander Riker suggests decompressing the main shuttlebay to move Enterprise out of danger, while Lt. Commander Data advocates using a tractor beam to push the other ship away. Captain Picard chooses Data's option; the maneuver is unsuccessful, and the other ship strikes one of Enterprise's warp nacelles, causing a critical failure and the destruction of the Enterprise moments later, at which point the loop restarts.

Initially, crew members are unaware of the loop. However, Dr. Crusher begins to hear voices as she goes to bed following the poker game. In a subsequent loop, the senior officers experience déjà vu and realize they can accurately predict the cards Data will deal during the poker game. Crusher uses a tricorder to record the voices and discovers they are the panicked commands and broadcasts of the crew. The senior staff work out that they are stuck in the loop; the voices they are hearing are their own from previous loops, just prior to the destruction of the ship.

They determine that the loop resets after the two ships collide but are unsure how to avert this outcome. Data suggests using his positronic brain to send a short message to himself in the next loop, which may help them avoid the collision. When they arrive at the anomaly and collide with the other ship, Data sends the message.

On the following loop, Crusher again has a feeling of déjà vu during the poker game, but when Data deals the next hand, all the cards are threes, followed by multiple sets of three of a kind. The number 3 begins appearing throughout the ship as the crew again determine they are stuck in a time loop. When they reach the anomaly and the ship appears, Data realizes that 3 stands for the number of command pips on Riker's uniform and quickly executes Riker's suggestion rather than his own. The tactic succeeds and allows Enterprise to safely clear the oncoming ship, ending the time loop.

The crew establish they have been trapped in the loop for 17 days, while the other ship, the USS Bozeman, has been missing for over 90 years. Upon making contact with the Bozeman ' s captain Morgan Bateson, Picard welcomes the Bozeman ' s crew to the 24th century as he plans to get Bateson caught up.