Dr. Scott is at the airfield supervising the test flight of the fourth plane to employ his experimental fuel diffuser. The pilot, Bob activates the diffuser and increases the throttle, but the electrical system shorts out and the radio goes dead. Scott and his assistant, chief control officer Dr. Court, lose contact with the pilot and can only watch from the control tower window as it plummets to earth.
Later, Court goes to a coffee house and meets with his real employer, Katrina. The Russian agent notes that the last three planes also crashed while testing Scott's diffuser. Court insists that the deaths were unavoidable when sabotaging the diffuser at Katrina's request, and she pays him his fee.
Diane finds Peter preparing to go see his old friend Scott at his plant. Peter insists that there's nothing wrong with the fuel diffuser and that he's known about the project since its early stages. He figures that Scott can use all the friends that he can get and leaves for the plant.
Scott is trying to comfort Bob's widow, Jenny, who blames him for the death of her husband and the other three pilots. As Scott's assistant Richards leads her out past Peter, Jenny continues to blame Scott for Bob's death. Peter realizes that Scott is ready to give up, and the scientist explains that the Ministry has cut off his funding. Despite his friend's reluctance, Peter volunteers to look around the plant and asks about Scott's present team. The scientist insists that he trusts them and there's no signs of sabotage, and then asks Court to show Peter around. Court waits until Scott leaves and then tells Peter that he must convince Scott to give up the project before anyone else is killed. He tells Peter that he hasn't found any signs of sabotage and that the diffuser must be responsible. Peter agrees to tell his friend but says that he still plans to look around.
Court goes to the business that Katrina uses as a cover and tells her that the Invisible Man is investigating the crashes. Katrina tells him to relax and let her assistant Stephan take care of the newcomer. Once Court leaves, she and Stephan drive out into the country. They park outside of Diane's home as she takes Sally to school. Peter has removed his bandages and wearing his invisible lab clothing, and doing household chores. Stephan is reluctant to shoot without a clear target so Katrina says she'll call Peter from a phone box and then Stephan can aim for where his head is.
When Katrina makes the call, Stephan fires but narrowly misses. Peter runs out to his car and drives away, and Stephan picks up Katrina and has her take the wheel. They drive after Peter and Stephan fires at the driver's seat. Peter's car goes out of control and plows into a river bank, throwing Peter in. Katrina and Stephan arrive and find Peter's congealing blood turning visible, and figure that he's dead and his body was thrown into the river. Before they can investigate further, two pedestrians run up and the Russians are forced to leave.
Wounded but alive, Peter swims to shore and calls Scott. He explains that someone is sabotaging the flights and that he was grazed, but survived since he drove from the passenger's side. Peter tells Scott to call Diane and tell her not to worry about anything she reads in the paper, and to keep the news of his survival secret.
Court meets with Katrina, who tells him that Peter will no longer be an issue. The control officer then goes to the plant, unaware that Peter is following him. Peter then goes to see Scott and tell him that he's going to run a bluff. He has Scott make a press announcement that he'll be performing one last test. Scott agrees but insists that he'll fly it himself rather than put another pilot at risk. When they learn about the announcement, both Court and Richards disagree, but Scott insists. Court then calls to tell Katrina that he'll sabotage that plane as well, and Katrina informs him that she and Stephan will be watching from the airfield.
The next day, Court completes the final inspection on the plane and the diffuser. However, once he's alone he sabotages the diffuser control box. Once Scott boards, Court wishes him luck and leaves the plane, while Katrina and Stephan watch from outside the airfield and listen in on the control tower radio frequency. In the cockpit, the invisible Peter tells Scott that he's aboard and has him call Court back on board to make a last-minute check. When Court comes into the cockpit, Peter locks the door behind him. Scott begins takeoff procedure and tells Court that he can help him on the flight.
Ignoring Court's protests, Scott takes the plane up and begins maneuvers to test the diffuser. Court finally draws a gun and tells Scott to land, but Peter easily disarms him and explains what he did. Peter forces Court to confess over the radio, and Katrina and Stephan overhear him. They drive away only to find the police waiting for them based on Peter's earlier warning. Meanwhile, Peter tells Scott to turn on the diffuser for the test. When Court warns how dangerous it is, insisting that he told the truth, Peter tells him that he watched as Court sabotaged it earlier, and undid the tampering.
The test is a complete success. Once Scott lands, he and Peter take Court back to the plant where the police are waiting to take Court into custody. The rest of the staff arrives and they and Peter offer Scott their congratulations.