On his first day at an Outward Bound centre, 16-year-old Terry drops his torch in a cave, so instructor Alex goes back for it. Terry is left alone for two hours but when Alex returns, he believes he's only been gone for ten minutes with no recall.

Alex remembers nothing of the previous day; however, Gordon tells Terry he believes him. Terry decides to go down the pot but when he visits Granny Powell for the key, she is acting odd. When Alex arrives, he barely knows Terry.

Terry follows Alex and Kurtz. Alex lies on the road, a car stops and Kurtz kidnaps Dr Andrew Jensen before driving off to the Powell house. Terry believes only Gordon can help but he convinces him that he works for the Ministry of Defence.

Gordon gets Terry to believe he works for the M.O.D. and was assigned to protect Dr Jensen. He learns Jensen was kidnapped to get the codes for an orbiting satellite. Terry is driven to the farm but soon makes a break.

Terry finds Alex, who still doesn't know who he is. Mr Richards puts him with Dr Jensen, who fills in some key details and gives the missing codes.

Terry is taken for treatment and gives up part of the code. When Jensen kicks up a storm, Terry flees. He is found by a passing motorist who takes him to the police.

Terry wakes in hospital, unable to remember even his name. After vanishing for 36 hours, Dr Jensen returns to work. Mr Buckle finds Granny Powell acting oddly. Terry recalls the Jensen Code just as Richards and Gordon arrive.

Richards removes their unwanted memories from Terry, but after a visit from Ron and Jacko, his story doesn't add up. He returns to the Centre but is picked up by Dr Jensen, who triggers Terry's memories.

Terry returns to the centre and manages to convince Mr Buckle and learns Alec and Granny Powell also suffered memory loss. Buckle gives him a map but warns him time is short. Jensen is questioned about his two missing days.

Terry returns to Granny Powell's and receives a call from Buckle, warning him stay away. When Gordon arrives, Terry attempts to bluff and allows to take him to Wilmers Deep where he leaves him. Richards attempts to sell the codes.

Granny Powell informs Buckle that Gordon took Terry down Wilmers Deep. Informing Mountain Rescue, he starts making his way down. Terry knows where he is but has to find the mineshaft before the water level rises.

The Rescue Squad don't believe Terry will come out alive. Buckle and Alex find the main shaft looking for pockets of air where Terry could be. When Terry comes to, he has a complete memory of the events but can he change the codes?

Jensen's assistant Miss Howard manages to postpone the Space Lab launch, fearing Terry is right about the stolen codes. Jensen admits he planned for his own kidnapping and used Terry to thwart the kidnappers getting the real codes.