This is one of the better written episodes of Superman. It begins with John Corben's (cf. Last Son of Krypton) jail escape after finding out that he got a rare fatal virus. Luthor planed his escape and convinced him to undergo a surgery procedure that would give him a body made of metallo with kryptonite as its source of energy.
Corben delivers a very realistic portrait of power-hungry man than becomes delighted with his new abilities, but as soon as he realizes that he won't feel any more, he panics and wants to be transferred back into a human body.
Corben: I can't even feel a kiss... What did they do to me?
Corben's concern with feelings and pleasure shows how human he really is. In spite of being a criminal, he is not as ruthless as Luthor is, looking to no means to achieve his goals. Corben wouldn't trade his humanity to get his revenge, but Luthor gave Corben's away – his supposed loyal employee – to get Superman out of his way.
There is an ironic contrast here: Corben is the one who is supposed to be a monster, but he proves to be as human as anybody else. However, Luthor shows to be even more ruthless and evil than previously, which makes him the real monster, as it turns out that he was behind the whole scheme.
The fact the Corben's virus was induced makes his fate even more cruel and tragic, which culminates with a disturbing a chilling scene where we see his shadow tearing his fake skin and clothing off, thus rejecting his fake human appearance and accepting what he is:
Corben: It's all fake. A FRAUD! There's the reality! The metal behind the man! It's all I am now! It's who I am... Metallo.
Malcolm McDowell's voice acting is superb, and makes Metallo one of the most charismatic villains in Superman. He is the heart of this episode, and is what makes it work. We can't help feeling sorrow for his misfortune fate. The musical theme used for Metallo also brings a twisted and disturbing effect to the episode.
This episode also has some witty scenes, for example when an old woman biking past Clark calls him "Slowpoke." Also, when Lex's date admires the size of his... yacht. These kind of deceiving comments make this show a bit more pleasing for a mature target. It was also nice to see Lex in some trouble for a change, as he was being chased by a shark.
The animation was pretty good, particularly the effect of the kryptonite illuminating the darkness on the lower decks of Luthor's yacht.
Every line in this episode seems to have a deeper meaning, specially the ironic comment that Corben makes to Dr. Vale in beginning:
"The trick is to never underestimate the value of a good friend, and always keep your mouth shut."
One can't help noticing the irony of this remark, since Luthor betrays and uses him.
The episode ends with the idea that this time Lex might have biting off more than he can chew. In the process of destroying Superman, he might have created his own nemesis. The very last scene wraps up the whole disturbing and chilling tone of the entire episode, as Corben slowly walks on the depths of the ocean floor towards his revenge.