this last episode of the Star Trek prequel features two very special guest stars: Jonathan Frakes and Marina Sirtis.

We are keeping mum on story details right now, but Frakes and Sirtis are indeed playing their Next Generation personas Riker and Troi. As co-executive producer Manny Coto mentioned in a recent interview, the finale does not involve time travel — the most we'll tell you here is that it does involve the Holodeck. [my bolding]

Another guest star in this episode is Jeffrey Combs, getting blue one last time as the Andorian "Shran."

During the eight-day principal photography schedule (one day longer than usual), most of the shooting took place on standing NX-01 sets, but there were new sets constructed to represent 24th-century locales. Among them: Ten Forward, the Observation Lounge, Troi's Quarters, and TNG-style Corridors. We will also see scenes in the Hologrid, but those were shot in front of a greenscreen for rendering later.

...

Throughout the eight days of shooting, Frakes was at various points dressed in a TNG-era uniform; as an NX-01 crewman; as a MACO; and ... as one other person who perhaps should remain a surprise.

The finale does not imply that the whole series has been seen from the Holodeck. In fact, what they show on the Holodeck is one specific mission that Riker is watching while trying to decide whether to tell Picard about the Federation-cloaking device from the TNG episode, "The Pegasus". What he watches on the Holodeck is a mission actually four years after the current timeframe that Enterprise is at in their storyline.

StarTrek.com - March 18, 2005

In the highly-anticipated Enterprise series finale, Captain Jonathan Archer (Scott Bakula) is involved in an Engineering accident with a ruptured Space Modulator, causing him to become exposed to a dangerously high dose of Illudium Q-36. His exposure causes him to experience recurring real-time hallucinations which ultimately result in him being relieved of command. In Sick Bay, Archer falls into a comatose-like state. Unable to effectively treat Archer to return him to consciousness, Dr. Phlox (John Billingsley) begins to suspect that his patient's condition may be due to the hallucinations rather than vice-versa. Phlox calls upon the assistance of Commander T'Pol (Jolene Blalock) to perform a mild-meld so as to inject a third party into Archer's mind and to slow his progressive loss of grip on reality.

Within Archer's mind, T'Pol observes the Captain interacting with various characters from the past and future, all of whom seem to be intent on luring him towards a mysterious blue swirling light source. Believing that her observations are purely passive, T'Pol is shocked when she is addressed directly by a character named Rear Admiral Albert Calavicci (Dean Stockwell). In a conflict between alternate realities, Calavicci provides convincing evidence that T'Pol, Phlox, and the entire Enterprise universe are figments of Archer's imagination, and that Archer is actually a scientist from the late 20th century who suffered an accident during a time-travel experiment.

After a series of flashbacks and flashforwards, including a holodeck scene involving Star Trek: The Next Generation's Captain William T. Riker (Jonathan Frakes) and Counselor Deanna Troi (Marina Sirtis), T'Pol realizes that there is no way that Archer could have "memories" that include the Federation's future. Thus, she accepts that Calavicci's story must indeed be true. She exits the mind meld, returning to the Enterprise's Sick Bay. Archer awakens suddenly, glances around the room and smiles broadly. He reaches out to T'Pol, who takes his hand and whispers "good bye" just as Archer's body enveloped in an ball of blue electricity.

Link: Blue Electrcity

Archer finds himself in a small chamber bathed in blue light. He moves toward the door which opens upward, revealing Calavicci holding a handlink. Al enters the chamber, extends his hand, and says, "Welcome home, Sam".