Edgar Scoville and David Vincent meet with Senator Robert Breeding in his Washington, D.C. office. They warn him that his friend, Arthur Koy, a senior advisor to the National Security Council, is either an alien or a "fellow traveler," as Breeding puts it. Scoville and Vincent claim that Koy is always in the area whenever a saucer lands.
Breeding says, "Mr. Vincent, I have lived my life with people who know that the enemies of the Republic are under every bed and behind every curtain." He accuses Scoville of being the real menace and kicks the two of them out of his office. Breeding's secretary notices a briefcase in the office and rushes to the elevator before Scoville and Vincent get on. Scoville already has a briefcase, so she asks Vincent if he left one in the office. Just as he denies having any briefcase at all, a blast comes from Breeding's office. They rush back and find Breeding dead.
Scoville and Vincent are taken to the office of Assistant U.S. Attorney Andrew M. Hatcher, who has asked an old flame who is a newspaper reporter named Joan Seeley to sit in on the interview. He asks Scoville and Vincent who blew up the Senator. They don't know. He reads the secretary's transcript of their meeting and twists it to make it sound as if they had a grudge against Breeding for rejecting them. Hatcher is particularly interested that they told Breeding about a secret organization they belong to called the Believers. He then lets them go.
Seeley visits the house where James Boland, Vincent and Scoville are staying. Boland is suspicious of Seeley, but Vincent decides that whether she is working for the press or Hatcher, she should see what they are doing. They show her a communications/computer room in the basement where they have been tracking the aliens. They also show her a film of NSC advisor Arthur Koy exchanging papers with a man they know to be an alien. They note that this is the first time the alien has given Koy something in return, an envelope. Seeley goes back to try to convince Hatcher not to prosecute Scoville and Vincent, but he has made up his mind to present what he thinks he knows to a grand jury. Although he doesn't tell Seeley, Hatcher finds out that the bomb that killed Breeding was made of an explosive exclusively manufactured by Scoville; however, he realizes that Scoville could say that anybody could have bought that explosive. Hatcher wants more evidence, and his prayer is soon answered in the form of Dr. Stanley Frederickson, who says he is a member of the Believers and wants to testify against them.
Boland, Scoville and Vincent find out that Koy is leaving for Paris that night. Before he gets away, Vincent and Seeley break into his home and wait for him. When he arrives, he opens a safe and takes out the same envelope that the alien was shown to give him on the surveillance footage. Vincent demands the envelope. Koy tosses it on the floor, and when Vincent goes to pick it up, Koy goes for a gun. Vincent shoots him, and Koy glows and burns up, convincing Seeley that aliens are real.
The next day, Frederickson goes before the grand jury and says that the Believers decided beforehand to murder Breeding if he did not help them go after Koy. Seeley leaves the room and goes to brief Vincent who tells her that he has never even heard of Frederickson. He suddenly realizes that the aliens must have known for some time the names of all the Believers. Meanwhile, Boland has partly decoded the documents and what he has so far suggests that the invaders are preparing for a final assault on humanity. Seeley then goes to Hatcher and invites him to dinner. Over dinner, they reminisce about their bygone relationship and agree to rekindle it at Seeley's apartment, but she has a surprise for him: Vincent and Scoville are there, and they show Hatcher the documents they got from Koy, also telling him that Koy died and burned up. Hatcher is extremely agreeable and says he will back off, but he immediately gets on his car-phone and tells his assistant to have the murder of Koy and other conspiracy charges added to the indictment.
When the Believers are indicted the next day, warrants sworn for their arrests, and Seeley goes to warn them. They tell her that they now think that the aliens intend to use an ultrasound weapon that requires three simultaneously operated transmitters to send sound waves that will kill every living thing on the planet. The computer they have in their basement is not adequate to crack the alien code further to find out the exact time of the attack and the locations of the transmitters; so Scoville suggests they go to a brand new facility he has built in Maryland and use the computer there. As they leave the house, the police arrive. While Scoville creates a diversion that leads to him being wounded, Boland and Vincent drive away. Seeley goes back to work on Hatcher, but he still doesn't believe. He takes her to meet Frederickson at a safe house, but when they arrive, nothing is left of Frederickson but some ashes on the bathroom floor. Seeley insists that the ashes lend credibility to the Believers' story, but Hatcher says it is just a trick and that Frederickson must have climbed out the bathroom window. Reports come in that all of the indicted Believers are either in custody or dead except for Boland and Vincent, who have been spotted in the vicinity of Scoville's new research facility in Maryland. Hatcher orders the state police to look for them there.
Meanwhile, Vincent and Boland crack the aliens' code and learn that the main transmitter is in the basement of a well-known D.C. hotel. Seeley arrives at the Scoville research facility and warns Vincent and Boland that authorities are aware of their location, but before they can get away, the police arrive. Seeley gets in Vincent and Boland's car and leads the police away while Vincent and Boland take her car. Seeley crashes the car.
While Vincent and Boland talk their way into the hotel basement, posing as telephone repairmen, Hatcher visits Seeley in the hospital where she tells him she understands that he could not believe even though he knew he was wrong. Hatcher still says he is right but he now seems less sure.
Vincent and Boland fight with alien guards, killing them, but Boland is killed, as well. Vincent opens a heavy door and finds the transmitter. He fires a gun at the transmitter which explodes. Apparently, the aliens' plan cannot work unless all three transmitters are online.
Later, Vincent meets Hatcher at the hospital. Hatcher says that Seeley died. He is now convinced that there really is an alien threat, and he promises to help the Believers in the future.
- This is the last episode of the series. Roy Thinnes later returned as David Vincent in the 1995 TV Movie.
- Peter Mark Richman as Andrew Hatcher (as Mark Richman)
- Susan Oliver as Joan Seeley
- John Milford as James Boland
- Kent Smith as Edgar Scoville
- Robert H. Harris as Dr. Stanley Frederickson
- Alex Gerry as Senator Robert Breeding
- Mary Gregory as Breeding's Secretary
- Burt Douglas as Alien #1
- Lincoln Demyan as Secruity
- Ernest Harada as George - Waiter
- George Robotham as Hotel Engineer