An unauthorised attempt by Captain Hikaru Sulu in 2293 to take the USS Excelsior through the Azure Nebula into Klingon territory to free Kirk and McCoy from their trial.
The incident is seen in the Voyager episode Flashback, through the memories of Tuvok, who was a young ensign on the Excelsior's science staff. It takes place during the events of Star Trek VI, just after the Praxis explosion and the arrest of Kirk and McCoy.
Sulu decides that loyalty to his old shipmates outweighs his orders and sets course for Qo'noS, hiding in a nebula. Tuvok formally protests. A Klingon ship commanded by Kang challenges them, and Sulu ignites the nebula's sirillium gas to disable it. More Klingon cruisers then attack, and the Excelsior is forced to withdraw. Lieutenant Dimitri Valtane dies in the fighting.
In 2373 Tuvok and Captain Janeway relive these events in a mind meld while searching for the source of a dangerous false memory.