Justice League Unlimited
Season 1, Episode 10: Dark Heart
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Most of the League battles with powerful alien nanotechnology but, realize that the waves of self-replicating robots cannot be simply destroyed. The diminutive Atom shrinks himself to stop the robots from the inside.
All Episodes - Season 1
Episode 1
Initiation
Episode 2
For the Man Who Has Everything
Episode 3
Kid Stuff
Episode 4
Hawk and Dove
Episode 5
This Little Piggy
Episode 6
Fearful Symmetry
Episode 7
The Greatest Story Never Told
Episode 8
The Return
Episode 9
Ultimatum
Episode 10
Dark Heart
Episode 11
Wake the Dead
Episode 12
The Once and Future Thing (1): Weird Western Tales
Episode 13
The Once and Future Thing (2): Time, Warped