Tattooed Teenage Alien Fighters from Beverly Hills

The Rat (1x13)

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Cerebula returns and raises up the temperature of the Earth, so Nimbar summons the sentinels. When they enter the portal, Gordon takes Swinton's rat through the portal with him and then fights Cerebula. After a short fight they return back through the portal, but Gordon doesn't come back as Gordon. Instead he comes back as a rat inside Swinton’s rat cage.