An airplane is flying overheard while in the control tower, Dr. Scott tells the pilot, Bob, to prepare to test his fuel diffuser which is designed to expand maximum flight range. Bob increases the throttle and Scott warns him it’s where the previous three pilots ran into trouble. The pilot activates the fuel diffuser and has electrical trouble. Scott loses contact with the pilot and they can only watch from the window as the plane crashes.
Later, Scott’s chief control officer Dr. Court goes to a coffee house and meets with a woman, Katrina. She directs his attention to a newspaper with a headline about the plane crash, the fourth one to occur testing Scott’s fuel diffuser. Court insists that he had to kill the four men to guarantee that the fuel diffuser test was unsuccessful, and Katrina slips him the agreed-upon sum for his sabotage efforts.
Peter is at home packing to go see Scott at his plant. When Diane wonders what he can do, Peter says that he’s known about the project since its early stages and figures that Scott can use all the friends that he can get. He insists that there’s nothing wrong with the fuel diffuser and takes off once Diane reminds him to put on his bandages.
At the plant, Scott tries comfort Bob’s widow Jenny, but she blames his mad scheme for the death of her husband and the other three pilots. Scott has his assistant lead Jenny out but she continues to insist that Scott killed her. Peter arrives and hears the tail end of the conversation, and says he’s here to help. Scott is ready to give up given all the people who are blaming him for the four deaths. Peter assures him that there isn’t anything wrong with the fuel diffuser, but Scott tells him that the Ministry has cut off all grants. Determined despite his friend’s depression, Peter asks Scott to take him around the plant and asks how long he’s had his present team. Scott insists that he trusts his team and there’s no signs of sabotage. Despite that, Peter insists on taking a look around.
Scott asks Court to show Peter around. Once they’re alone, Court says that Peter must convince Scott that his work is a complete failure. The control officer assures him that he’s carefully examined all of the wreckage and found no signs of sabotage. Peter says that he’ll tell Scott that the fuel diffuser doesn’t work, but tells Court that he still plans to take a look around.
Court promptly goes to Katrina at the timber company her country owns as a front. Her assistant Stephan lets Court in and the scientist tells Katrina that Peter Brady is investigating the crashes and will be back at the plant the next day. Katrina tells him to relax and let Stephan dispose of Peter, and then she leaves with Stephan for a drive in the country.
Stephan and Katrina park outside of the Wilson home and Stephan prepares a rifle. Meanwhile, inside Peter is doing dishes and wearing his invisible clothing, and Diane takes Sally to school. Stephan and Katrina approach the house through the woods and spot Peter’s approximate location by the dishes seemingly floating through the air. Stephan refuses to shoot without a clear shot, insisting he has to kill him with a single bullet. Katrina suggests that she call him from a phone box and then they’ll know exactly where Peter’s head is when he picks up the phone.
As Katrina makes the call, Stephan goes to the car and draws a bead on the phone. When Peter answers, Stephan fires but misses. Peter runs out to the car and drives away past Katrina, who realizes that her assassin has failed. Stephan picks her up and she takes the wheel. They drive after Peter, who loses ground when he’s forced to pull around a slow-moving tractor. Katrina drives her car closer and Stephan opens fire. Peter loses control of the car and it plows into a river, throwing the driver into the water. They find congealing blood turning visible and soon realize that he’s in the river. However, two pedestrians hear the commotion and Katrina and Stephan are forced to leave.
Peter swims to shore and gets to a phone box. As he calls the Scott plant, the police pull up and go to Peter’s car. When Scott answers, Peter tells him that the pilots died because of sabotage, and that he only survived because he drove from the passenger’s seat. He tells Scott to call Diane and tell her not to worry no matter what she hears about Peter, and to tell no one else that he’s alive.
Later, Court meets with Katrina again and asks if the reports that Peter is missing and presumed dead are true. She tells him that he won’t have any more trouble for Peter. Court goes to the plant, unaware that Peter is invisibly hiding nearby. Peter then sneaks into Scott’s office and says that he’s going to run a bluff. He tells his friend to call the press and announce he’s conducting one final test. Scott agrees but only if he flies it himself rather than risk another pilot, and Peter agrees.
Scott’s staff warns him against taking the plane up but the designer insists that he’ll take the plane up. Court sends a message to Katrina saying that he’ll sabotage the fifth plane the same as the last four and Katrina tells Stephan that the two of them will be watching from hiding.
The next day, Court conducts the final inspection and signs off on it, but then once he’s alone he sabotages the diffuser control. Scott boards the plane and Court assures him that everything is fine and wishes him luck. While Scott prepares for takeoff, Stephan and Katrina watch from their car and tune into the airfield radio. Peter reveals to Scott that he’s aboard and tells his friend to call Court back on board to check the gauges. When Court enters the cockpit, Peter locks the door behind him and Scott prepares to take off, saying Court can help him with the test. Court panics but Scott ignores him and lifts off.
Scott takes the plane through high-G maneuvers to thoroughly test the diffuser. Realizing that time is running out for him, Court draws a gun and tells Scott to land the plane. Peter announces his presence and easily disarms Court, and tells him to reveal what he did. Court’s confession goes out on the radio and Katrina and Stephan hear it. They hastily drive away but Peter has already notified the police, who cut them off and take them into custody. Peter tells Scott to turn on the fuel diffuser. When Court insists that he’s telling the truth, Peter tells him that he undid the sabotage.
Once the test is a success and Scott lands the plane, they take Court back to his office where the police are waiting to take him away. Peter and the rest of Scott’s staff offer their congratulations to the engineer.