The Joining has launched its attack on Earth and the key to defeating them is Wayne Industries' satellites. As Batman and Martian Manhunter attempt to penetrate Wayne Towers, Alfred encourages Batgirl and Robin to disobey Batman while the battle for Earth begins.
Tropes for this episode include:
- Actually Pretty Funny: The Joker finds it most amusing that the invasion has prompted an Enemy Mine between the police and supervillains.The Joker: This town's gone mad! [laughs]
- Crazy-Prepared: Per Lucius, Bruce always insisted on Wayne technology having a secret failsafe so that it can be shutdown remotely if any of it ever got into the wrong hands.
- Darkest Hour: The Joining attacks multiple cities across the planet.Batgirl: It's the end of the world.
- Defiant to the End: Through the robot head, the Joining observes that humanity is this.
- Didn't See That Coming:
- No one has ever managed to defeat the Joining before. Their only response to the heroes prevailing is, "How unusual."
- Batman knew more about the Watchtower than he had let on and hacked its systems. The other Leaguers are rather surprised.
- Enemy Mine: GCPD teams up with the Arkham Rogues to repel the alien invasion.
- Evil Versus Oblivion: Batman's enemies help the GCPD fight the Joining, even Joker, because terrorizing Gotham is his job.
- Foreshadowing: J'onn keeps reminding Batman of the importance of teamwork and trusting partners. In the last scene, he invites him to join a new team.
- From Bad to Worse: As the group watches the Joining trash Gotham, J'onn tells him this is happening across the planet.
- Good News, Bad News: The Joining used Wayne Industries technology as part of their invasion, but they didn't know that Bruce always made sure to have a failsafe installed so as to shut it off if need be. The problem is that the heroes can only send the shutdown signal to the entire fleet from a facility that's under heavy guard.
- "Hell, Yes!" Moment: The people's reaction around the planet, as the Joining ships explode and crash.
- Hidden Heart of Gold: Batman ordered Robin and Batgirl to stay out of it not because he thinks they'd get in the way, but out of fear of losing them.
- Hoist by His Own Petard: The robot head points out the Joining is simply taking Earth's own technology, improving it, and then using it to wipe them out. Batman, however, is able to use this against them because they used Wayne Industries technology, and he had a failsafe installed that can allow him to shut down the invasion fleet.
- Ineffectual Loner: Lampshaded. Batman is aware he can't make the necessary adjustments to block the signal and fend off attackers at the same time. Robin and Batgirl reveal they're there and insist on being back-up.
- It Won't Turn Off: The robot head of Lucius suddenly starts talking and bragging about how the Joining cannot be stopped. Batgirl suggests someone unplug it, but Robin says it is unplugged. The head is able to tap into the power of the Joining remotely.
- Loophole Abuse: Robin and Batgirl formally quit being Batman's partners so that he can't order them to stay away.
- Oh, Crap!: Robin and Batgirl over learning they're dealing with a full-blown Alien Invasion and not just attacking robots.
- Pragmatic Villainy: When attacking one of the robots, Joker says that terrorizing Gotham is his job.
- Race Against the Clock: Batman has mere minutes to get the signal sent before the invasion fleet begins the final destruction of Earth. The signal is sent just as the blasts are about to go off.
- Secret Secret-Keeper: Batman's known about the construction of the Watchtower for months.
- Seen It All: Batgirl questions how Alfred can be so calm in the face of J'onn entering the Batcave.Alfred: I keep house for a man who dresses like a bat and works in a cave. At this point in my life, there's little that fazes me.
- Sequel Hook: To Season 5.
- Stunned Silence: Robin and Batgirl over seeing J'onn in action.
- Team Spirit: The theme of the episode.J'onn: The entire world rebuilds towards a new tomorrow, a tomorrow that would never have come if not for one man who wanted to make a difference. You saved your planet, Bruce.
Batman: I didn't. We did. - This Cannot Be!: Batman's reaction to the Joining attacking Gotham.
- Wham Episode: The Justice League shows up at the end, and Batman reveals that he's been aware of their actions for some time before joining the team.
- Wham Line: J'onn expresses hope that Batman can expand his circle and be a team player.Batman: I already have two partners. Don't need a third.
J'onn: Actually, we were hoping you would join us. - While Rome Burns: While watching the invasion destroy Gotham, both Alfred and Lucius reminisce about helping Bruce through his youth. After recalling he'd just told Dick and Barbara to defy Bruce's orders to not get involved with him and J'onn stopping the Joining, Alfred and Lucius decide they can help, too.
- You Are Not Alone: When inviting Batman to join the Justice League, J'onn pitches it as saying it's made up of those like the two of them and dedicated to the same cause.
- You Have Outlived Your Usefulness: Once the Joining has everything they desire, they order their ships to form rings above each major city and begin a countdown that will destroy the entire planet.
