Professor Robinson is caught in a matter transfer emission ray, and subsequently ends up in another dimension where he is confronted by a version of himself and Don. However this realm is the opposite of his own dimension, the anti-matter world, where everything in reality is twisted. The evil John returns to the family and attempts to take over the Robinsons and take over John's life. Will he get away with it, or are Will and the Robot able to defeat him, and restore the world to it's natural order.

Une expérience qui tourne mal envoie John dans un monde d'anti-matière et renvoie son double maléfique à sa place. Will, Smith, et le Robot tentent de sauver John, qui est gardé par deux autres doubles, ceux de Don et du Robot. Le faux John est près à tuer tout le monde pour conserver sa nouvelle liberté hors du monde d'anti-matière.