While this episode is far from serious (even by Lost In Space standards), it is memorable.
Dr. Smith finds an alien device that produces cyborg clones of himself and naturally he decides that he will use his army to rule the universe. Of course things do not work out as he had planned but it is still entertaining to see all of the Zachary look-alikes.
The highpoint of the episode however is when Will falls into the cyborg machine and comes out with Dr. Smith's head. It isn't an exact match but is close enough to be truly weird and hilarious.
There are better Lost In Space episodes but this is one you will remember.