(covers information from several alternate timelines)
In that year, the Enterprise-C responded to a distress call from the Klingon outpost at Narendra III, which was under attack by four Romulan Warbirds. Castillo was on the bridge during the firefight and survived as the Enterprise-C was badly damaged and drifted through a temporal rift, arriving in the year 2366. He was rescued by the crew of the ship's immediate successor, the USS Enterprise-D, which was nearby the rift at that time. The Enterprise-D immediately started repairs, with Castillo acting as liaison while Garrett recovered in sickbay.
Captain Jean-Luc Picard informed them that the Federation was now at war with the Klingon Empire, and was on the brink of defeat. It was eventually determined that the Enterprise-C's journey through the rift had resulted in the creation of this alternate timeline, and that, if they wished to restore the proper flow of history, the Enterprise-C would have to return through the rift, in the middle of the battle with the Romulans. Despite certain death, Castillo, among many of the crew, agreed that they should return.
As the crew of the Enterprise-D assisted their counterparts in making repairs, Castillo made the acquaintance of Tasha Yar, the Enterprise-D's chief tactical officer. She taught him about the advances made during their time, what life was like, and they became close. Castillo having to leave caused a certain amount of distress for both Yar and himself, but they both knew what he had to do.
Before the Enterprise-C could depart, however, both ships came under attack by a Klingon Bird-of-Prey, resulting in Garrett's death. As the highest-ranking crewmember left alive, Castillo took command of the Enterprise-C, and prepared to take the ship back through the rift. He was surprised when Yar decided to join him and his crew, having learned that she had died a meaningless death in the other timeline. However, he welcomed her and put her on tactical. Together, they were able to take the ship through the rift and were successful in repairing the timeline. (TNG: "Yesterday's Enterprise")
Richard Castillo was played by Christopher McDonald.
The final draft script of "Yesterday's Enterprise" described Richard Castillo thus; "Castillo is ruggedly handsome, with jet black hair and fiery eyes."
The costume for Castillo included three five-year service pins, meaning that Castillo had been in Starfleet since at least 2329.
In the Star Trek Online mission "Temporal Ambassador", it is revealed that the Enterprise-D's battle with the Klingons in "Yesterday's Enterprise" caused the Enterprise-C to emerge from the temporal rift in the Azure Nebula in 2409, rather than Narendra III in 2344. The alternate timeline remains in place, and the Federation has fallen to the Klingon Empire (which in turn fell to the Dominion years later). The Enterprise and her crew - including Castillo - are captured by the Tholian Assembly and held aboard a base in the Azure Nebula, where the player must help them escape and reach the temporal rift so they can return to 2344.