The Leopard Mask Gang are ???.
A bunch of leopard-dressed gangsters who used duplicates of the Metal Men to do their bidding. The Metal Men are out shopping, when they hear police sirens. Platinum forms a gleaming web across the road and stops a gang of villains. The enraged villains emerge guns blazing and the unique robots form fantastic shapes as the stop the gang, but the leader of the Leopard Gang manages to get away with the loot.
The Evil Gang Are Activated Driving away he decides to kidnap the Doc and force the metal men to do whatever he wants, sneaking into the lab he sees the Doc explaining to his date that he has created a second group of metal Men for the army with evil responsometers so the army can devise ways of stopping them. On hearing this the evil leader fights with the Doc and overcomes him savagely. He finds their responsometers and switches on the gang and leads them to start a crime wave.
Laden with shopping the Metal Men run into their evil counterparts and the leader of the Leopard Gang who use the element of surprise to overcome them. Triumphantly the evil gang steal the jetaway to use as a getaway vehicle. The Metal Men recover and make their way to the lab where they find the Doc has got amnesia from his fight with the gang leader and he does not know who he is or whom the Metal Men are. Tina uses this to get the Doc to treat her like a real girl.
The rest of the metal Men pair up and in individual jetaways go after the metallic rogues. Tin and Nameless are the first pair to find the gang and they attack but are quickly overwhelmed as the evil Iron crushes Tin and Nameless only just catches him before he hits the deck. Alerted by Tin Gold and Lead spot the gang, they knock down the evil Gold and Mercury, but are again overwhelmed by the rest of the gang.
The Leopard Gang leader then decides to destroy the Metal Men headquarters, Doc and Platinum engage them, but Doc is hit in the head by a bullet. Tins goes mad with grief entangling the criminal robots in a gleaming web and she throws herself and them into the smelter destroying the gang. The leader of the Leopard Man is arrested and Doc who's memory has been restored by the bullet sets about restoring the Metal Men.
Doc puts the Metal Men into the recovery room, Platinum awakens and tries to kiss the Doc after a wonderful dream.