Originally aired: October 19, 1969 on ABC
Writer: Bob Mitchell, Esther Mitchell
Director: Harry Harris
Show Stars: Don Marshall (Dan Erickson), Deanna Lund (Valerie Ames Scott), Heather Young (I) (Betty Hamilton), Gary Conway (Captain Steve Burton), Stefan Arngrim (Barry Lockridge), Don Matheson (Mark Wilson), Kurt Kasznar (Commander Alexander B. Fitzhugh)
Recurring Role: Kevin Hagen (Inspector Kobick)
Guest Stars: Leonard Stone (Sgt. Eason)
Production Code: 4707
While evading a SID sergeant, Steve ends up beneath a toadstool that dumps spores on him while Dan draws the sergeant away. Shortly thereafter, Mark is assaulted by an unseen assailant and Betty vanishes. While Dan and Mark look for Steve, he returns to the ship and asks Valerie to help him rescue Betty. The radio isn't working, so they leave Barry behind. Barry discovers the radio is unplugged, and contacts the others, then goes after Steve.
It turns out Steve is undergoing psychotic hallucinations due to the spores, and Kobick finds out about it. Steve captures Valerie and Barry, then grabs Fitzhugh and imprisons them all in an abandoned real estate office. Fearing they're trying to sell him out, he contacts Kobick and tries to make a deal. While Kobick tries to trace the call and overhear their conversations via Steve's lost radio, Dan and Mark continue the search. Steve meets with them and lures Mark off into a trap.
Seeing both Mark and Chipper display brief bits of violence from small traces of the spores, Dan puts it together and avoids another trap by Steve. As Kobick closes in, Steve and Dan fight it out and Steve snaps out of it just before killing his friend. The group manages to make their escape just as Kobick arrives.