Star Trek: Voyager, season 4, episode 14
Message in a Bottle
Summary
The Doctor's program is sent to an advanced Starfleet vessel via a vast ancient communications network, but he soon discovers that only he and the ship's own EMH remain to fight against Romulans who have taken over the ship and are attempting to return to Romulan space with it.
Summary from Wikipedia, CC BY-SA 4.0.
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