A dump truck filled with sand makes its way along a coastal highway and into the city to a construction site. Workers stand ready with shovels as the truck unloads. When the sand pours out onto the ground, an unconscious Dan Williams tumbles out too.
At the Governor's office, McGarrett and Governor Jameson discuss the impending arrival of Chinese official Lin Mai-Lu the following afternoon. Lin is mainland China's minister of commerce, but his trip is strictly unofficial. He's visiting Hawaii at the invitation of his old wartime friend, Honolulu resident Norton Hummel. While McGarrett is in the Governor's office, Ben Kokua calls with the news about Danno.
At the hospital, McGarrett has Ben take Danno's clothes to Che Fong for examination. The truck driver and the loading workers know only that Danno wasn't in the truck when they loaded the sand. During the four-mile drive to the construction site, the driver stopped only at traffic signals after he came into the city. Somewhere along that route, Danno either climbed or was thrown into the truck. Throwing seems unlikely, because the truck's sides are nine feet high.
Fortunately, Danno has only a mild concussion and will be released the next day. However, he can't remember anything except that it was his day off, and he got up early and got dressed to go out. The next thing he knew, he was in the hospital.
Che finds horse dander, horse blood, and burrs on Danno's jeans, sneakers and torn shirt. At one point, Danno had been riding a horse. Later, he ran on foot through heavy brush. Ben checks the local riding academies, and eventually finds Danno's car parked at one stable. However, the owner is gone for the day, and no one else there knows anything about Danno.
McGarrett, Chin, and Duke visit Hummel at his home to discuss the security arrangements for Lin's visit. Hummel has several excursions planned for his friend, but McGarrett nixes all of them except a boat ride. Chin notices a living room photo of Hummel's son, in Marine Corps uniform.
After Danno is released from the hospital, Ben takes him to the stable to try and jog his memory. The owner remembers Danno, and says he couldn't have been thrown because the horse would have come home. She's still missing. A few moments later a passing truck backfires, and Danno immediately dives for cover.
Although Danno is out of the hospital, McGarrett sends him home on sick leave. Eventually Danno remembers that during his ride, he came upon a bunker and saw two men going inside. One carried two boxes; the other carried what looked like a body. Danno identified himself as a police officer, and one man fired a gun at him. Danno fled, and the men gave chase in a jeep.
McGarrett, Chin and Ben join Danno at the now-empty bunker. Danno recalls seeing a boat inside. The four of them retrace his flight. Along the way they find jeep and horse tracks, part of Danno's torn shirt, bloodstains on a plant, the body of the horse, and a bullet casing. The trail ends at the top of a hill overlooking the coastal highway.
The hill is too steep to climb down on foot. Danno had been trapped. He then remembers that the dump truck came along at that moment, and he escaped by jumping into it. When he hit the sand, he struck his head on the side of the truck.
Back at the office, the others try to help Danno remember more details about the boat, the boxes and the body. The boat had been the size of a sailboat. The boxes were wooden crates labeled "Danger - High Explosives." The body was wrapped in a blanket, so Danno couldn't see it clearly.
A fingerprint found in the bunker belongs to Harvey Durko, who beat the rap on a charge of industrial espionage in Los Angeles. He was registered at a local hotel. On one occasion he took a cab from his hotel to a cottage on the highway where Danno jumped into the truck.
Chin, Ben and Duke go to the cottage to pick up Durko. He fires shots at them from inside, then flees through the back door. As they give chase, a second man escapes unnoticed. After Durko wounds Duke, Ben tries to disable Durko but ends up killing him instead.
The second man later catches up with Hummel and informs him that Durko is dead. However, Durko had finished the job. The man gives Hummel an electronic homing device hidden inside a cigarette case. Hummel assures him that the Chinese really will "blame the Navy. They consider us an imperialistic country dominated by the military." The man leaves, and Hummel turns his attention back to the grave of his son. Warren Hummel had been killed in action during the Korean War. He was only 19 years old.
With Danno's second would-be killer still on the loose, McGarrett sends Danno to stay at his own home until they sort everything out. McGarrett and Ben leave to accompany Hummel and Lin on the boat ride. As an HPD officer drives Danno to McGarrett's home, they pass a clothing store with mannequins in the front window. Danno then remembers everything, and contacts Chin to ask about Durko's background. Durko had been an electrical engineer, once employed at a company that made guidance systems and automatic pilots.
A harbor patrol boat takes Danno to Hummel's yacht, which has stopped in the water due to a mechanical problem. Danno boards the yacht and tells Steve that the men he saw are planning to kill Lin, with the boat he saw in the bunker. A small U.S. Navy motorboat appears in the distance. Danno quickly tells Steve that the motorboat is loaded with dynamite, and the sailors on board are mannequins. Guided by an automatic pilot installed by Durko, the fake Navy boat is going to ram the yacht and blow everyone aboard to bits.
McGarrett sees through his binoculars that the sailors are indeed mannequins. He and Danno open fire, igniting the dynamite and destroying the boat before it reaches the yacht. His original plan foiled, Hummel pulls a gun and fires at Lin. McGarrett shoots and wounds Hummel. After ordering Ben to contact the Coast Guard, McGarrett asks Hummel, "Why?" Glaring at Lin, Hummel angrily replies, "He killed my only son."