When the TARDIS materialises on a strange planet, the crew discover that the beach is made of sharp glass and the water of acid. Is something being kept in, or kept out? As they head off to explore, one by one they are captured by the robe-clad "Arbitan" (George Colouris) who explains that he is in search of a set of keys that will operate a machine that will offer him freedom - especially important as the encroaching "Voord" are never far away and he is not getting any younger. The "Doctor" (William Hartnell) doesn't think this is any of their business, but with "Arbitan" holding the TARDIS key, the four must don some wristbands that enable instantaneous spatial travel and go search. There now follows a series on stand-alone adventures that take them to the city of "Morphoton" - where nothing is quite as it seems; then a jungle world where the hungry plant life has it's eyes on four human-sized snacks. Thirdly, there's the most interesting of the stories as "Ian" (William Russell) and "Barbara" (Jacqueline Hill) follow the ever disobedient "Susan" (Carole Ann Ford) into a system of caves that are the home of the menacing "Ice Soldiers" before, finally, the fourth key has to be obtained despite considerable danger to "Ian" who has been quite neatly fitted up for a murder, and who's trial requires the "Doctor" to use all his own wits and guile to track down the real culprit before his friend goes the way of the dodo. The concluding episode in the series sees them return to collect the key to their time machine, only to find that there's been a change of management on "Marinus" and the marauding "Voord" are poised for their final confrontation. The episodic nature of these adventures works fine, with each character getting a shot at the limelight whilst Hartnell holds the narrative together well. I usually found that three companions for the "Doctor" was too many - especially the annoying "Susan", but here it works to advantage with some decent stories, writing and some baddies and puzzles that provide an engaging challenge for everyone.