In the British countryside, Peter waits with a military team. They check the sky and watch a cloud drift overhead with the prevailing wind. Peter tells everyone to get down as the cloud approaches a nearby bomb site, and a few seconds later the bomb goes off. Captain Barry Finch congratulates Peter on the success of his new bomb detonator.
Back at the military base, Peter and Barry review the detonator with Captain Betty Clark, Barry's partner on the project. Peter demonstrates that the detonator responds to any interruption of light shining on it. He makes sure that the two military officers have what they need from him and depart. Betty and Barry proceed to install the detonator to a mine, which has the standard trembler switch on nit.
The next day, Barry meets with his superior, General Martin, who is with the ministry man, Lloyd. Barry explains how the bomb works and assures Martin that it will work, and Lloyd asks why it is set for light. Martin explains that the mines will be used in desert warfare only, and will go off at night. The general explains about how he lost men to British mines, so they want mines that will detonate at a predetermined time: nightfall. Once they're done, Martin tells Barry to prepare a demonstration for the next day at 3 p.m. Barry goes outside to where Betty is waiting and assures her that everything went fine.
The next day, the military and the ministry people go out to the test site. Betty and Barry prepare the mine in a pit while everyone else takes cover behind a wall of sandbags. Martin informs the ministry people that they will have a plane pass over head and use a smoke trail to block off the light to the mine at a precise time. As he finishes planting the mine in the pit, Barry invites Betty to dinner that night to celebrate and she accepts. Barry then calls Martin on a field phone and sets the trembler fuse with a 30-second delay, while the general tells the plane to proceed. The sergeant helping Barry drives off, unaware that a line is still running from his Jeep to the pit. A support comes loose and the pit caves in on top of Barry, who is trapped and trying desperately to hold up the boards so they don't land on the mine.
Realizing that something is wrong, Martin runs over just as the bomb goes live. Barry warns Martin that he can't move without setting off the mine. The general has his assistant call off the plane's approach and then brings in an emergency rescue crew. However, the pit is too narrow for anyone to get inside without casting a shadow on the mine. Martin realizes that there is only one man who can get into the pit without casting a shadow: Peer.
When the military reach Peter by phone, he immediately agrees to help and drives to the test site as fast as he can. However, Betty has calculated that they only have 18 minutes until a cloud passes overhead, setting off the mine. She offers to go down and save her friend, but Martin refuse to let anyone endanger themselves in a hopeless cause. Peter finally arrives and gets a toolkit, but warns Martin that while he knows all about the detonator, he had nothing to do with the mine itself. Betty offers to coach him from nearby since she does, and Peter has her get as close to the edge of the pit as she can to call out instructions to him.
Peter climbs down into the trench and starts trying to clear away the dirt. Barry warns that he's close to passing out from the strain and that he can't hold out much longer. Working desperately against time, Peter follows Betty's instructions, opens the side of the mine, and cuts the two wires. The next step is to remove the cover to get inside the mine proper. The cloud fast approaches and Barry tells Peter to get out, but his friend refuses. The plate sticks and Barry talks him through finding and disengaging the release lever.
A formation of jets approach the area and Martin gets air control on the line and calls to have the jets rerouted. They pull away and leave the area, while the clock counts down from one minute. Betty tells Peter to unscrew the spring detonator and pull it straight out: any contact with the side will set the mine off. Peter manages to pull the detonator out safely, and Betty and Martin run over to dig Barry out.
Once Barry is freed, he tells Martin that he's fine and insists on getting the mine in working order and setting it off just a little later than planned. A relieved Betty hugs Barry and then kisses the invisible Peter in thanks before they start digging the mine out.