This is one of my favorites of the entire series. The Robinson's are on trial for space crimes. Prof. Robinson, Don, Will and eventually Dr. Smith go on trial.
On trail there are flashbacks to the early b&w episodes to determine who was responsible for the various "crimes". Of course Dr. Smith is worried since he knows he is responsible for just about all the crimes.
There is perfect balance between the comedy and serious nature of the situation. Dr. Smith on trial is superb. The ending is great.
Interesting note: The female Robinsons do not go on trail. I suspect CBS did not want that visual.