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The Robinson family starts to explore the planet where they've crash-landed, but still face the threat of Dr. Smith (Jonathan Harris) and the robot, which he has set out to kill anyone it finds. Will Robinson (Billy Mumy) interrupts its attempt to kill him by getting it to play chess until Smith can be persuaded to deactivate it. They find out that the planet supports at least one potentially dangerous life form: a hybrid planet/animal micro-organism in the soil that can take root in and take over a larger host, and which also seems to be the start of an evolutionary chain. A hint of what lies at the other end of that chain comes when the robot blows its circuits over something it encounters while investigating a strange noise. Meanwhile, Professor Robinson (Guy Williams) and Major West (Mark Goddard) discover that because of the planet's long elliptical orbit, they're due for a lethal deep-freeze and need to move south -- and run right into a 50-foot Cyclopean giant (Lamar Lundy). Will rescues them, and they prepare to escape the coming cold; but first the professor must find the missing Penny (Angela Cartwright), and flies across the landscape using his rocket-pack to locate her. They arrive back just in time, leaving the unwilling Smith behind in the Jupiter 2 to try and survive the cold on his own.