The Jupiter 2 receives a transmission signal from an interstellar relay station based on Planet Delta. The male voice sending the message identifies himself as ‘Bartholomew’. The Robinsons note Bartholomew sounds quite young.
The Jupiter 2 lands on Planet Delta. The Robinsons are greeted by Bartholomew who looks about 16 – and his similarly youthful entourage. They present each of the female Robinsons with a bouquet of flowers.
Bartholomew explains Planet Delta is an advance ‘processing’ station designed for ‘screening’ people before they move onto Planet Gamma. He mentions Penny and Will will be accommodated separately from their parents. When Professor Robinson says he’d prefer they stayed with them he is told it is just regulation.
So begins Penny and Will’s ‘Adjustment Period’. They are each placed in separate study rooms. They learn geometry while loud rock music plays over speakers. Both are told they’ve been around their ‘olders’ too long and will learn better once they are free of their ‘former associations’. When Will uses the word ‘Gosh!’ in conversation, Bartholomew remarks, “That’s a really in word… in the squares dictionary”.
Another hippie-styled teenager named Edgar offers Penny and Will what he calls ‘memory cones’. He says when you light them up they smell like incense but also boost memory and help you learn faster. “All you gotta do is let go of Mama and Papa” he advises Penny and Will. “Just tune out those senior citizens and you’ll be able to breeze through your adjustment course”.
The next time John and Maureen see Penny and Will they have changed. Penny tells her father to “Cool it!” But she’s only getting started. “Will’s a big boy now. And I’m a big girl. So why don’t you just go back with the rest of the elders and leave us do our thing. You don’t belong here. Olders are out now. We’re in”.
Will persuades Penny to escape with him back to the Jupiter 2. Unfortunately John and the rest of the family have come under the influence of some kind of brainwashing of their own and have no memory that Will and Penny are their children. They leave in the ship without them, but not before Will gives his father some memory cones as a parting gift, saying “You’ll need this more than I will”.
Once out of orbit, the robot tells Professor Robinson that only now he is free of the magnetic impulses of Planet Delta is he able to determine what happened. He tells John his memories of Will and Penny were blanked out by an alien smoke screen.
The robot advises him to light up the memory cones. They give off a red smoke and John is able to clearly hear his children’s voices. The Jupiter 2 immediately alters course to return to Planet Delta.
Detecting their approaching ship, Bartholomew forces Will to transmit a message to the Jupiter 2 telling his father and family they are not wanted and to go back where they came from. “One thing we don’t need is any help from you. Quit making with the hero bit and reverse course.” Penny is coerced into adding, “Look man, we don’t groove to the same vibration. Chow man”.
Meanwhile on Planet Delta Bartholomew admits to Penny he and his kind cannot grow old. This is why they need the Robinsons. He prepares Will and Penny to undergo a blood transfusion so that their ageing chemistry can be transferred to the alien teenagers who inhabit the planet. John and Don arrive just in time, rescue Penny and Will and depart the planet.