Director: James Goldstone
Writer: Robert Bloch

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Lieutenant Uhura: Nichelle Nichols

Nurse Christine Chapel: Majel Barret
Dr. Roger Korby: Michael Strong
Ruk: Ted Cassidy
Andrea: Sherry Jackson
Dr. Brown: Harry Basch
Matthews: Vince Deadrick
Rayburn: Budd Albright

Stardate 2712.4

The Enterprise arrives at Exo III and the crew looks for the "Pasteur of archeological medicine", Dr. Roger Korby, sister Chapel’s fiancée. They are not sure whether he still is alive. Dr. Korby welcomes them and requests that only Kirk and his fiancée come down onto Exo III. In underground caverns they find their way and meet Ruk and Andrea. They introduce them to their lives in the caverns and refer to "The Old Ones". Further Korby explains that long-dead Exoites have left equipment behind whit which he constructed androids who look and act like humans. Ruk and Andrea, the two androids Kirk and Chapel met look extremely real to them. They learn that Ruk already was a product of "The Old Ones." 

First Christine Chapel wonders why Dr. Brown, a co-worker of her fiancee does not recognize her – until she finds out that he, too, is an android. Korby shows them how easy it is to duplicate humans, including their minds, knowledge and feelings and duplicates Kirk. The only chance for Kirk - who himself realizes how difficult it is to distinguish the real human from the android - to create a difference is to plant wrong information into his mind, before the process begins. 

Sister Chapel has a conversation with Kirk and only finds out who he is when he tells her that he does not eat because androids do not need food. The duplicate enjoys her confusion. Korby's long- term plan is to replace key people in the Federation with androids and he needs the Enterprise to achieve his intentions. The android–Kirk is beamed on board to arrange for the others to come along. Meanwhile, Kirk manages to convince Ruk that Korby wants to get rid of him and Korby that he has become more machine than man by all his mechanical implants. 

A struggle begins and all duplicates die, Dr. Korby, too. Quickly Spock beams down with a landing party. The short encounter with the Kirk duplicate has made him sure it is not his captain. "Something bothering you, Spock?" "Frankly, I was rather dismayed at your use of the term 'half-breed', Captain. You must admit it is an unsophisticated expression." "I'll remember that Mr. Spock – the next time I find myself in a similar situation."