"Sentries of the Last Cosmos" is the twenty-third episode of the second season of Batman Beyond. It depicts the arrival of a trio of teenagers literally enacting the events of a VR game. The top three players of Sentries of the Last Cosmos are called together by Simon Harper, the creator of the game. Convinced that the game is not just a game but reality, they are granted actual flaming swords and flying disks. Believing that they are actually helping out the side of good, they begin sabotaging parts of Gotham. Worst of all, these players are so good that even Batman is hard pressed to stop them.
Out on a distant planet, a small group of soldiers riding on flying disks carrying energy swords rush to evade pursuing space ships. One is forced to crash but the other proves to be piloted by a much more skilled flyer. One of the warriors is almost done in but another warrior flies in and destroys the ship at a great risk to himself. With the second ship down, the warriors remove their helmets revealing themselves to be Max, Terry and Corey Cavalieri playing a VR Game: Sentries of the Last Cosmos. Corey is the last to remove his helmet and is proud to discover he's earned the all-time high score. Unfortunately for him, he finds that reality isn't quite so kind to him. Unbeknownst to him, however, a real sentry is watching him. Back at school, Corey muses over creating new scenarios for the game and brushes off Max's thought that he's too obsessed with Sentries of the Last Cosmos. Just then, he receives an e-mail from Simon Harper, the creator of the game, inviting him to Harper's private estate, Sentrycroft. Corey is overjoyed by this news and readily accepts the invitation.
Upon reaching Sentrycroft, Corey is awed by the paraphernalia normally found only in the games—vehicles, weapons, uniforms. He meets two other players, Dempsey and Burfid also in awe of what they've found. Harper, wearing the "robe of the wise one" greets them and congratulates them as being the three top-scoring players in all of Gotham. They reply, "To us, Sentries of the Last Cosmos is more than a game". "Yeah, it's our whole life!" Simon reveals a great secret: the world depicted in the game represents an actual conflict; there really is a corp of Sentries, and the game is actually a test to find new recruits worthy of joining. He invites the boys to join the ranks of the Sentries.
Elsewhere, Batman patrols the skies of Gotham and learns from Bruce that there's been a break-in at the Hall of Records. Batman investigates and finds the three new Sentries—complete with uniforms, flying disks, and energy swords—preparing to destroy the records, claiming that the databank has been infected by "the Dark Regent". Batman confronts them and when it becomes apparent he's there to stop them, the three quickly incapacitate him and destroy the records.
Returning to school, Terry talks to Max. When he repeats the Sentries' words that they were on "a sacred mission from Wise One", Max remembers Corey calling Harper the "Wise One". Max and Terry go to Corey and ask about what he saw at Sentrycroft but he refuses to tell them anything. They both realize something is wrong, because normally Corey wouldn't be able to stop talking about the place.
That night, Batman investigates Sentrycroft. Unfortunately, a security system releases several probes that attack. Batman destroys the probes and is met by Harper. He questions Harper about the Sentries but Harper denies any involvement. After Batman leaves Simon calls in his Sentries and gives them their next mission: the Dark Regent is on Earth disguised as a man known as Eldon Michaels.
Having unearthed Michaels' name in connection with Harper's, Batman goes to investigate his apartment. Once there, he is menaced by a shadowy man with a gun. Batman disables him with a batarang and is surprised to see that the man is Michaels, who is harmless and was only trying to defend himself.
Batman asks Eldon why Harper is after him, to which Eldon replies, because Harper is "a two-faced Janus, a pirate and a charlatan". Before he can show Batman the evidence of what he's talking about, the Sentries arrive and attack. Batman tries to fight them off, but he's unsuccessful and Eldon is captured. In spite of his failure, Batman manages to retrieve the envelope that Eldon tried to give him.
The Sentries take Eldon to Harper who is angry that they didn't kill him. The Sentries claim they couldn’t, as it would violate the Sentries' Code of Honor. Simon has no such aversions and starts to attack Eldon with lighting bolts thrown from his hands, claiming that Eldon has plotted against him. Bewildered and scared, Eldon says it was only a lawsuit he filed.
Hearing this, the Sentries are confused. Batman then appears and shows them the envelope, which contains the original story for Sentries of the Last Cosmos. Batman explains that Eldon wrote the story, and partnered with Harper to produce it as a video game. However, Harper didn't want to share the immense profits and popularity from the game, and so he cut Eldon out. What the Sentries were actually destroying were the original contracts between Eldon and Harper. Harper attacks Batman with his lightning, while the Sentries start to read the story. Batman manages to subdue Harper, who calls desperately to his Sentries for help. However, now realizing that he stole the story from Eldon—and lied to them about the Sentries being real—they turn on him.
Terry returns to Bruce, who pities Corey and his friends for looking to a video game for something to believe in. This leads Terry to wonder aloud why Harper went to all the trouble to eliminate Michaels, since he was already plenty rich and didn't owe Michaels that much money. Bruce speculates that Harper cared less about money than losing his fans' worship.
However, the players aren't done with the game. They return the original draft to Eldon and ask him questions about the game, claiming that he's the "real Wise One". Liking the idea, Eldon invites them in and begins to tell the story.
- Sentries of the Last Cosmos would reappear in "Curse of the Kobra" and The Zeta Project episode "Cabin Pressure", in which Bucky and Agent West both agree that the latest version (quite likely, Eldon Michaels') was bogus so they quit.