When Mariner asked the Krulmuth-B portal to send her, Rutherford, Tendi, and Boimler to the moment when they could make the biggest impact on history, she meant, like, to the time period where they could help their crew the most. But they've ended up on board the Titanic! Yes, THE Titanic, and that's just the tip of the iceberg. The time traveler who's responsible for a deadly wave of Federation rewrites is still at large, and the Lower Deckers are now further than ever from stopping them.