A Wells-class timeship from the 29th century, commanded by Captain Braxton, that polices the timeline and recruited Seven of Nine to stop a temporal saboteur.
The USS Relativity is a Wells-class timeship operated by the Federation in the 29th century. Captain Braxton commands it, and its crew works to protect history by spotting temporal incursions and dispatching agents to repair them. The ship can send people across centuries and pull them back out again.
In the Voyager episode named after the ship, the Relativity crew discover that a bomb has been planted aboard Voyager in the 24th century. They recruit Seven of Nine, whose Borg implants help her move through time, and send her on repeated trips into Voyager's past, including the day it was launched from Utopia Planitia. The saboteur turns out to be a future version of Braxton himself, driven to madness by years of correcting Voyager's temporal interference, and he is taken into custody.
The Relativity shows the timeship side of Star Trek's temporal policing, echoing the wider struggle over the timeline seen in the Temporal Cold War. Its mission of guarding history from within the future made it a model for later depictions of Starfleet's temporal agencies.