During Kirk's second year of command in 2266, Brent, who served in the sciences division, was on the bridge on stardate 1512.2, helping at the weapons sub-system station when the Enterprise engaged the First Federation. (TOS: "The Corbomite Maneuver") On stardate 1535.8, Brent was working at the bridge's life support station when Charlie Evans, with psionic powers from the non-corporeal Thasians wreaked havoc with the Enterprise and her crew. (TOS: "Charlie X")
Brent was present in a corridor during a few of Kirk's (and one time Commander Spock's) speeches to the crew. (TOS: "The Corbomite Maneuver", "Balance of Terror", "The Menagerie, Part I", "Assignment: Earth")
Later, when the Enterprise was in orbit of planet M-113, Brent and Bobby stepped aside as Uhura exited the turbolift, located next to the ship's botany section, while under observation by Crewman Green, who was in actuality the M-113 creature, who upon seeing Uhura, transformed into another crewman. (TOS: "The Man Trap")
On stardate 1704.3, Brent relieved Lieutenant Uhura at navigation after Spock had her relieve Lieutenant Kevin Riley, on stardate 1704.2 because Spock had thought that Riley was inebriated and Spock had relieved him of duties, with Riley leaving the bridge. On stardate 1704.4, while at navigation, Brent plotted a course so the Enterprise would not be dragged into the gravity well created by the rapidly deteriorating orbit of the planet Psi 2000 as the planet was destructing naturally for its life span. Brent did not contract the Psi 2000 polywater intoxication. (TOS: "The Naked Time")
Near the end of 2266 on stardate 1709.1, Brent who was serving in the command division, attended the wedding of Robert Tomlinson and Angela Martine before it was canceled by Romulan sightings. Later on Brent, who was then serving in the sciences division, once again, was seated at the engineering sub system station on the bridge during the investigation of the destruction of Earth outposts by the Romulans. (TOS: "Balance of Terror")
In 2267 on stardate 2124.5, Brent, once again in the command division, was on the bridge when Lieutenant Hikaru Sulu then Kirk were abducted by Trelane. On stardate 2125.7, when Chief Medical Officer Leonard McCoy, Lieutenant Carl Jaeger, and Lieutenant DeSalle beamed down to the rogue planet Gothos to find the missing Kirk and Sulu, Brent replaced DeSalle at navigation. On stardate 2126. 1, Brent left the bridge on the turbolift with Lieutenant Leslie, Lieutenant Hadley, and an engineering crewman when Kirk, Sulu, McCoy, DeSalle, and Jaeger returned from Gothos and Brent was not on the bridge when Trelane appeared there suddenly. (TOS: "The Squire of Gothos")
On stardate 2821.5, Brent, returning to the sciences division, was at the bridge's main science station relieving Spock, as Spock was in command of the crew on board the shuttlecraft Galileo as it was investigating the Murasaki 312 quasar. From stardates 2821.7 to 2823.1, Brent was relieved at science when Uhura led the search, and then, acting temporarily as Enterprise's main science officer, helped Kirk with the rescue of the crew from the planet Taurus II. But Brent continued to stay on the bridge acting as Uhura's main assistant by coordinating all of the efforts of the entire bridge crew in sciences, engineering, environmental and communications. (TOS: "The Galileo Seven")
On stardate 2945.7, Brent, once again in the command division, sat at navigation when the Enterprise encountered an ion storm. (TOS: "Court Martial") On stardate 3025.3 Brent once again manned navigation, as a member of the sciences division. (TOS: "Shore Leave")
On stardate 3087.6, Brent, who returned to the command division, was on the bridge working on repairs at the life support station when the Enterprise was preparing to leave the orbit of an iron-silica planet when the whole universe, including the ship, winked out of existence. The shock-wave from this winking-out of the universe caused Brent to be unsteady and he had to kneel and hold on to the control panel at life support to keep from falling. Unfortunately Brent may not have learned his lesson regarding holding on when the Enterprise was being rocked or under attack. (TOS: "The Alternative Factor")
On stardate 3113.2, Brent returned to the sciences division once again, and was on the bridge at the weapons sub-systems station when the Enterprise got dragged into a black star with a high gravitational attraction. To avoid a disaster, Lieutenant Commander Montgomery Scott, chief engineer, used a slingshot effect to try and pull away. Unfortunately it sent both everyone on the bridge falling to the floor, including Kirk, Spock, and Brent, and threw the ship back three centuries to Earth, over Omaha, Nebraska's US 498th Airbase Group on July 13, 1969. After a nearly disastrous three days in July 1969, the Enterprise again used the slingshot effect using Sol to return to 2267. When the ship got to 2247, the drag again became too much to keep gravitation on the side of most of the bridge crew, but unfortunately this time Brent was the only one to fall to the floor of the bridge. Spock later helped him stand up when they returned to 2267 on stardate 3114.1. Brent was able to stay up right during the second jolt by holding on to the railing of the bridge. (TOS: "Tomorrow is Yesterday")
Some time later, he manned the life support station when the Enterprise was rocked by time displacement waves as the ship was in orbit around the Guardian of Forever's planet. (TOS: "The City on the Edge of Forever")
Brent was on the bridge helping out at engineering sub-systems station when Khan Noonien Singh took over the Enterprise on stardate 3142.8. Brent passed out after the life support to the bridge was cut, and was seated in a prisoner's row with Leslie, Hadley, and others, forced to watch Khan's murderous spectacle on the viewscreen. (TOS: "Space Seed")
On stardate 3417.4, Brent was helping Dr. McCoy packing up and beaming up the possessions of the colonists on the planet Omicron Ceti III due to the bombardment of deadly Berthold rays. But when Lieutenant DeSalle introduced to Dr. McCoy the pod plants, Brent was one of the first Enterprise crew members to be infected by the spores. This was the first and only time that Brent deserted the Enterprise and mutinied against his commanding officer, Kirk. When he and the other crew members were free of the influence of the spores Brent returned to duty. (TOS: "This Side of Paradise")
In 2268, Brent temporarily relieved Spock at the bridge's main science station when Kirk spoke to Bela Okmyx and couldn't repress giggling when Kirk tried to explain to Okmyx how he and McCoy and Spock were going to beam down to Sigma Iotia II. Brent's giggling was so bad that the rest of the time Spock was on the planet Sigma Iotia II, he had to be relieved of duty at the main science station by Lieutenant Hadley. (TOS: "A Piece of the Action")
On stardate 4307.1, Brent assisted Dr. McCoy and Nurse Christine Chapel in sickbay, when McCoy ordered stimulants administered to the crew, as the Enterprise encountered an unexplained "zone of darkness" of space, devoid even of stars. Brent had to keep fellow crew members steady as Chapel administered the stimulants, due to many of them feeling devoid of energy and faint. On stardate 4308.8, Brent continued to assist McCoy and Chapel, in sickbay as even more of his fellow crew members started to feel even more lithe less to the point where McCoy told Kirk that both the whole crew and the ship were dying. On stardate 4309.4, when the "zone of darkness" was discovered to be an enormous single-celled organism, that fed off energy enough to kill, and Spock in a shuttlecraft and the whole ship moved in closer to investigate and then to destroy the organism the Enterprise was again rocked violently. Brent, still in science blue, stationed at the bridge's life support station was as with the rest of the bridge crew thrown by the rocking. Unfortunately for Brent, he was thrown the worse and farthest as he was thrown all the way from life support station to the floor, on his back, in front of the turbolift. (TOS: "The Immunity Syndrome")
On stardate 4657.5, Brent returned to the command division, and was working in environmental engineering when the Kelvan Milky Way Expedition hijacked the Enterprise to return to the Andromeda Galaxy. Kelvan Drea suddenly appeared and put Brent into temporary "frozen" and motionless stasis. On stardate 4658.9, the Kelvans neutralized and reduced Brent into a dehydrated porous cuboctahedron solid the size of a Human fist, composed of Brent's base minerals which represented the "distilled" essence of Brent's being. The Kelvans considered Brent one of many non-essential personnel. Brent was reconstituted after Kirk, Spock, McCoy, and Scott, the only four members of the crew not neutralized, regained control of the Enterprise. (TOS: "By Any Other Name")
On stardate 5423.4, Brent returned to the science division, and was stationed at main science on the bridge when Spock was in temporary command and Hodin from the planet Gideon's Council reported that Kirk had not arrived, in beaming down, in the council chambers. On stardate 5423.6, a very somber (along with McCoy, Scott, and the rest of the bridge crew) Brent seemed confused, but persistent, and stubborn, as McCoy, Scott, and Uhura were that Spock stand his ground – with Spock concurring – that the Gideon Council and Hodin (as well as Admiral Fitzgerald not concurring), were hiding the truth as to where Kirk had disappeared to and Hodin was too evasive on the subject. (TOS: "The Mark of Gideon")
On stardate 5630.8, he was present in the corridor with a female yeoman when Kirk addressed "all ship's personnel", informing them that "clearance plans now in effect," and to "clear passageways immediately," as the recently arrived Medusan Ambassador Kollos was being transferred from the transporter room to his quarters. (TOS: "Is There in Truth No Beauty?")
Also in 2268, Brent appeared at Captain Kirk's memorial service, and was later on the bridge when Kirk briefly appeared to the crew from the interphase. A few weeks later, on stardate 5730.6, Brent, again in science blue, attended with Sulu, Chekov, and Hadley, in the recreation room, the speech by Lokai regarding how his people had been oppressed on his home planet of Cheron for many centuries. In the last few days of 2268, Brent, in science blue, beamed down to the Federation maintained Elba II asylum, for the criminally insane, to assist Dr. McCoy and an operations division lieutenant to administer a new medicine that potentially could eliminate mental illness. (TOS: "The Tholian Web", "Let That Be Your Last Battlefield", "Whom Gods Destroy")
A few days into 2269 on stardate 5725.6, Brent was walking in a corridor in front of the briefing room when Scott and Lieutenant Mira Romaine walked in. Later in 2269, on stardate 5832.6, Brent, in science blue, was stationed at the bridge's life support station when Dr. Sevrin, held up in a locked out with his followers from the rest of the ship in auxiliary control, applied ultrasonics to the Enterprise to knock the crew unconscious so Sevrin and his followers could steal the shuttlecraft Galileo II and head into Romulan space to get to the planet of Eden. Brent passed out from the ultrasonics in front of the life support station, although he recovered a short time later. (TOS: "The Lights of Zetar", "The Way to Eden")
- "The Corbomite Maneuver" (TOS Season 1)
- "Mudd's Women"
- "The Man Trap"
- "The Naked Time"
- "Charlie X"
- "Balance of Terror"
- "Shore Leave"
- "The Squire of Gothos"
- "The Galileo Seven"
- "Court Martial"
- "The Menagerie, Part I" (stock footage)
- "The Menagerie, Part II"
- "The Alternative Factor"
- "Tomorrow is Yesterday"
- "The Return of the Archons"
- "A Taste of Armageddon"
- "Space Seed"
- "This Side of Paradise"
- "Errand of Mercy"
- "The City on the Edge of Forever"
- "Operation -- Annihilate!"
- "Catspaw" (TOS Season 2)
- "The Changeling"
- "The Deadly Years"
- "Bread and Circuses"
- "The Gamesters of Triskelion"
- "The Immunity Syndrome"
- "A Piece of the Action"
- "By Any Other Name"
- "Return to Tomorrow"
- "The Ultimate Computer"
- "Spock's Brain" (TOS Season 3)
- "Is There in Truth No Beauty?"
- "The Tholian Web"
- "Let That Be Your Last Battlefield"
- "Whom Gods Destroy"
- "The Mark of Gideon"
- "The Lights of Zetar"
- "The Way to Eden"
Brent was portrayed by actor Frank da Vinci and was only named as such in "The Naked Time". This was also the only episode where the character of Lieutenant Brent spoke dialogue although he still didn't receive credit.
The novel A Choice of Catastrophes gives him the first name Clifford.