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Captain Archer points out the first Class M planet that Enterprise ever visited, which is a reference to 1st Season's "Strange New World" (#004). He also mentions his torture of an alien (054 "Anomaly"), his stranding of a helpless crew (071 "Damage") & covers his encounter with the Vulcan ship The Seleya (057 "Impulse") during his debriefing.
Trip references his sisters death, and the destruction of his hometown (052 "The Expanse"). Trip's relationship with T'Pol continues to be explored in this episode.
For her part, T'Pol references the fact that she has been ill recently; it's possible that she is referencing both her Trellium-D addiction (057 "Impulse" marking her first exposure), and her Pa'nar Syndrome (040 "Stigma").
Vulcan Ambassador Soval returns for another appearance during the debriefing scenes. He was last seen in an alternate universe in Season 3 (060 "Twilight"). He returns later this season (083 "The Forge").
Commander Williams makes his final appearance during this episode's debriefing; he was last seen in Season 2 (#049 "Regeneration").
Admiral Forrest also returns during the debriefing scenes; he was last seen in Season 2 (052 "The Expanse"). He appears again later this season (083 "The Forge").
T'Pol's mother, T'Les is introduced here. She reveals here that she chose to retire from the Vulcan Science Academy rather than face dismissal. She was under investigation by the Security Ministry, who claimed that she had taken restricted date from the Academy archives. T'Les explains that the Security Ministry's accusations were fabricated as a kind of retaliation towards T'Pol for her involvement in the mission that led to the destruction of the Vulcan's P'Jem Monastery (007 "The Andorian Incident") ; when T'Pol wasn't within their grasp, they struck out at her mother. They told T'Les that the inquiry would end if she chose to resign. T'Les returns later this season (084 "Awaking").
This episode marks the first appearance of Koss, who is T'Pol's husband by the end of the episode, but we've known about him, and his link to T'Pol since season 1 (008 "Breaking The Ice"); that he actually appears is a fantastic bit of continuity - and really helps strengthen the importance of the first 2 seasons, which were mostly kind of bland. Koss returns later in the season (083 "The Forge", 085 "Kir'Shara").
This episode also introduces Captain Erika Hernandez, who is the Captain of the new NX-02, the Columbia, which was mentioned last season. Erika mentions that while Archer was away one of the movies that was released was "Another World War III epic. It swept all the awards.". World War III has been mentioned several times in Trek but never actually depicted. In the Classic Trek episode "A Piece Of The Action" it was mentioned that the death toll for World War III was 37 million; in "Star Trek: First Contact" however the death toll was stated to be 600 million, though it's possible that the Next Gen crew had access to info that had not been revealed to Kirk's crew. Captain Hernandez & the Columbia return later in the season (091 "Affliction").
The Vulcan Priest who presides over T'Pol's wedding to Koss returns later in the season (085 "Kir'Shara").