This episode - the only one written by William Welch for season 3 - is a solid suspense tale as first Maureen Robinson mysteriously vanishes off the ship while in outer space and then one by one the other members of the Robinson party begin to vanish as well. It is fairly unique as one of only two episodes set entirely in space (the other being "Wild Adventure") and all the cast has at least a few lines in it - though with Will and Smith still getting the lion's share of the script. At least Smith's buffoonery is kept to a minimum and he even has a long scene as a sinister version of himself - more like his persona in the earliest episodes. Altogether, it's a highly recommended episode and one of the third season's best.
Now what's the bonus? It's a blooper that no one else ever seemed to notice. When John is trying to ascertain what is causing the disappearances he stops and asks Will, "Where was your mother when you saw her last?" Will replies it was by the sonic washer. But Will was confined to his cabin when his mother vanished early in the show, so he would have had no idea where she was before she disappeared. However, he was the only one with Penny just before *she* vanished - and by the same sonic washer that Maureen had been standing next to. In other words, John's line should have been "Where was your *sister* when you saw her last?" But in all the books and articles I've ever read about LOST IN SPACE, many which included lists of bloopers, no one else ever seemed to have caught this one.