“The Immunity Syndrome” is episode twenty-three of season two of Space: 1999.
The Alphans have discovered an Earth-like planet that looks like it can support them. A survey team has landed to evaluate it, and the commander has quarantined eating or drinking for now. No life has been found, but there’s a mysterious structure. The commander and Alan are looking at it and being told about it. Koenig calls Tony, who gushes about where he is, and explains what they found and that he wants everyone back.
Maya is in the Command Centre and she’s saying the planet is perfect. Dr Russell says that’s just raw data; they need to wait for the computer analysis. Maya finds the computer slow.
The other Alphan, Joe, who was with Tony, hears something and goes looking. He encounters a flashing light and clutches his head and screams. Tony goes to look and Joe tries to shoot him. They struggle and Joe ends up being shot himself. Tony is about to call it in when the flashing light happens again. He screams and runs. When the commander calls Tony, he crushes his commlock.
Dr Spencer tells Koenig he thinks Joe’s death was an accident. Two security guards return; they found Tony’s commlock half a kilometre away, away from base camp. Tony is still running and shoots a tree stump that startles him. On Alpha, Dr Russell is looking through Tony’s records when Maya enters.
On the planet, Koenig and two guards are following Tony. Alibe tells an Alphan off for eating fruit. Travis isn’t getting anywhere cutting into the door when Alan joins him. Alan suggests burning away the rock around it. This reveals an entry mechanism, but without power. However, there are solar cells and Alan and Travis decide to clear more of them away. Alan calls the commander and explains that once the door is charged, they will be able to get in. The commander says they’ve found Tony, but not got him yet. He wants Alan and Dr Spencer to pick them up in an Eagle.
Koenig has found Tony, and instead of doing the sensible thing and stunning him unawares, calls out to him. Tony tries to shoot the commander, they struggle and he tries again. Then collapses. Dr Spencer says he’s lapsing into a deep coma and needs Alpha’s medical facilities. They take off in the Eagle.
Travis tells a couple of men by a rock pool that the quarantine period is over. They drink the water. Then keel over, dead. Travis contacts Alpha to tell them and that he needs to speak to Koenig when his commlock fries. Maya contacts the Eagle, only for the Eagle communications to fry. The Eagle suffers other problems and will have trouble getting back to Alpha. Koenig uses his commlock to contact Maya, but that fries too. In the Command Centre, Maya fills in the doctor, then reports the planet’s atmosphere is changing.
Koenig is checking the Eagle’s systems before attempting to return to Alpha, but they’re corroded. They head back to base camp. On the planet, the man who ate fruit is now dead as well. One of the controls on the Eagle snaps off in Alan’s hand, then fire breaks out. The Eagle crashes and the commander is injured too. Alan heads to check out Eagle 1.
Eagle 1 is also corroded and won’t fly. Everything on the planet, including the atmosphere, is turning poisonous. Though Joe died from laser fire, he would probably have died from brain damage anyway, and Tony has the same injury. They have no communications with Alpha, no Eagles and if Alpha tries to send an Eagle, it will be corroded by the atmosphere, which is destroying metal. They do have a mysterious structure that might hold a clue.