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The Joining, Part 2 is the thirteenth episode of The Batman season 4.
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.
As the Joining begin their attack, Batman, Martian Manhunter, and Lucius Fox escape into Wayne Enterprise and escape to the Batcave via the emergancy exit. The trio reaches the Batcave where Lucius is attended to by Alfred and Batman tells Robin and Batgirl about the Joining's invasion.
As cities all over the globe are attacked and there is little that any military or police forces can do to stop it. Batman realizes that the Joining used Wayne Enterprise technology to build a hovercraft that had been taken earlier and the team figures out that the Joining has made a mistake as all Wayne Enterprise devises have fail safes so they can be shut off just in case. The robot Lucius head says that the Joining is unstopable and they are becoming stronger and will use earth technology to destroy the planet. At the same time, Commissioner Gordon calls upon Batman to help them as he is out of ideas after a rocket squad failed to destroy any of the Joining.
Batman and Martian Manhunter go off to Wayne Enterprise to send a signal to a satellight to activate the fail safe and destroy the Joining while Alfred, Lucius, Robin, and Batgirl stay at the Batcave. Martian Manhunter tells Batman he has to trust Robin and Batgirl to provide him with assistance because the Joining are linked and work together so to defeat them, the heroes must do the same. At the Batcave, Alfred persuades Batgirl and Robin to disobey Batman and help fight the Joining. The sidekicks go off to help Batman while Batman and Martian arrive at Wayne Enterprise and Lucius and Alfred deside to help in their own way.
As the invasion progresses, villains such as Joker, Bane, Penguin, and Mr. Freeze work with the police to protect their planet, a police officer assisting Freeze and Freeze himself saving Gordon from certain death. With the once cops and villains now working together, Joker remarks, "This town's gone mad!"
Robin and Batgirl head to Wayne Enterprise while Batman and Manhunter fight their way through the building to send a signal to a satellight but the signal is blocked and the Joining are breaking through to Batman. Batgirl, Robin, and Manhunter hold off the Joining while Batman hacks into the satellight while Alfred and Lucius assist from the Batcave. Batman activates the fail safe and the Joining are defeated.
All around the world, the Joining fall and rebuilding commences. Manhunter teleports Batman to a building which also holds Flash, Hawkman, Green Lantern, Green Arrow, and Superman. Batman joins but not before saying he hacked into the computers and knows that the base is really a spacestation.