From 2290 to 2293, Valtane served on the USS Excelsior under Captain Hikaru Sulu for its three-year mission cataloging gaseous planetary bodies in the Beta Quadrant. His living quarters was on deck 7, along with bunk-mate Ensign Tuvok and two others, who all worked during the gamma shift. (Star Trek VI: The Undiscovered Country; VOY: "Flashback")
On their return, the Excelsior was hit from a shockwave while Valtane was on duty. He located the source, Qo'noS' moon, Praxis, and found it had exploded. (Star Trek VI: The Undiscovered Country) Later, he learned along with all the others that Captain Kirk and Dr. McCoy were imprisoned on Rura Penthe for the murder of Chancellor Gorkon. As an eager young officer, Valtane expressed both approval and surprise regarding Captain Sulu's recklessness in attempting to rescue the two officers. He observed Tuvok's objection to Sulu's decision as well. Later, privately, Tuvok expressed his negative outlook on Humans to him. Valtane did not take any personal offense, however, and, at the end of the conversation, merely remarked that Tuvok's determination to resign his commission in the future would be "his loss".
Following Captain Sulu's abortive attempt to rescue Kirk and McCoy, the Excelsior came under attack while attempting to enter Klingon space. Gravely injured during the attack, Valtane, while he was apparently in a state of clinical death, unwittingly transferred a memory virus to his attendant crewmate Tuvok. (VOY: "Flashback") He was apparenly resuscitated, as he survived. Later, he was on the bridge of the Excelsior when it rendezvoused with the USS Enterprise-A at Khitomer. (Star Trek VI: The Undiscovered Country)
This character had no first name in Star Trek VI: The Undiscovered Country; it wasn't established as "Dimitri" until "Flashback". The name wasn't included in the first draft of the episode's script, but was in the final draft of that teleplay.
In the script of "Flashback" (both the first and final draft of the teleplay), the adult Valtane was consistently referred to as a lieutenant.
He was called Masoud Valtane in the novelization of Star Trek VI: The Undiscovered Country, as well as the novel The Fearful Summons (which was written by Star Trek VI screenwriter and character co-creator Denny Martin Flinn).
Peter David's novel The Captain's Daughter stated that Valtane was the Excelsior's first officer; the novelization of Star Trek VI also stated this. However, DC Comics had a story arc, collected in Tests of Courage, by Howard Weinstein in their second TOS volume that instead had Janice Rand as first officer.
According to his biography in the video game Star Trek: Starship Creator, Valtane was the son of Nikolai and Larisa Valtane. He was born in 2259 on the Beta Niobe colony, and entered Starfleet Academy in 2277. In 2280, while serving aboard the USS Malinche, he rescued two wounded crewmen from a pack of Denebian slime devils, armed only with a stick. In 2281, Valtane was assigned to the USS Gallipoli as a junior astrophysicist. In 2284, he was promoted to lieutenant and assigned to the USS Lamneth as chief stellar cartographer on a mission of exploration in the Gurava sector. In 2289, Valtane transferred to the USS Churchill as science officer. In 2291, Valtane wrote a scientific paper titled "Methods for Stabilizing Protomatter in High-Energy Matrices."