Star Trek: Voyager: Flashback

A hundred years before the Starship VoyagerTM was transported to the Delta Quadrant, Lieutenant Tuvok served under one of Starfleet's most famous officers: Captain Hikaru Sulu of the Starship ExcelsiorTM.
Now those days have come back to haunt him. While traveling through an uncharted nebula, Tuvok is besieged by recurring memories of his time with Captain Sulu -- repressed memories that may well kill him unless their source is determined in time.

To save her closest friend, Captain Kathryn Janeway follows Tuvok to the century-old bridge of the Excelsior during a desperate battle. There Tuvok, Captain Janeway, Captain Sulu and Commander Janice Rand must face a menace to galactic life unlike anything known before. . . .