Lifeline, a Brussels-based resistance organisation smuggling downed RAF fliers back to Britain, is endangered when Luftwaffe Major Erwin Brandt blows one of their safehouses. The Lifeline leaders, Lisa Colbert and café proprietor Albert Foiret, are suspicious when London sends Flight Lieutenant John Curtis, who had previously gone down the line with Lisa, to help co-ordinate their activities. Brandt, meanwhile, also finds assistance from Gestapo Sturnbannfuhrer Ludwig Kessler, transferred from Berlin to crack down on the evasion lines.

Albert and Lisa continue to be suspicious of Curtis, whilst Lifeline is stretched to its limit trying to deal with several evaders. The Germans capture one evader, Sergeant Walker, who has travelled down the line and is familiar with several of Lifeline’s personnel. Walker makes a desperate bid for freedom to avoid talking and throws himself down a staircase at Gestapo headquarters. As Walker recovers in hospital, Lifeline deliberate over whether he can be rescued from the Germans or if he must be silenced to preserve the security of the line.

The Jewish population of Brussels is being systemically deported according to colour-coded identification papers developed by the Germans. Curtis becomes involved with a Jewish family hiding out in the city and asks Lisa if Lifeline can help, but Lisa is unwilling to put the line, which is supposed to help RAF evaders not Jews, at risk. Curtis himself is still under suspicion by Albert, when Lisa’s uncle, bank manager Gaston Colbert, identifies Belgian banknotes he brought over from London as forgeries. Kessler obtains some of the notes, and realises that they are too accurate to be real and have been made by the Belgian treasury plates, which were evacuated to London before the German invasion. He believes that the notes will bring him closer to Lifeline.

An American airman has written a magazine article about his experiences being shot down in Europe and getting back home with the help of the evasion lines. British Intelligence has blocked the article’s publication, but Brandt has managed to obtain a copy from German intelligence based in America. Studying the article, he begins to deduce the probable route taken during the evasion and travels to the Pyrenees, where he enlists the aid of Malaud, a French policeman, to close in on the line. Meanwhile, Lisa is also in the area, taking some evaders down the very route Brandt is so close to tracking down.

Lifeline’s latest intake of evaders have been forced to leave a wounded colleague behind, hidden in a barn, but Lisa and Albert are unable to make a move due to heavy German activity in that area. The wounded evader, Sergeant Howson, is befriended and helped by a young boy, Jean-Paul Dornes, but Howson needs medical attention and Jean-Paul has to contact the resistance. Unfortunately, Jean-Paul’s mother Anna, who has raised him alone since her husband died in the fighting and is now involved with Emil Schnorr, a corporal from the local barracks, is sympathetic to the Germans.

Flight Lieutenant Peter Romsey, a RAF navigator, is one of Lifeline’s latest customers and London are anxious for his swift return – his technical knowledge of the Oboe navigation system is vitally important. Natalie takes Romsey and other evaders down the line, but Romsey is separated from the others and takes the wrong train, finding himself lost in the French countryside. As Lifeline tries to track him down, Romsey finds unexpected assistance from Hugh and Dorothy Neville, an expatriate English couple. A celebrated author utterly disillusioned with the war, Neville must decide where his true loyalties lie when Romsey turns up on his doorstep.

A British bomber crash lands somewhere outside Brussels, its entire crew is apparently dead. Kessler takes this as an opportunity to infiltrate Lifeline and assigns Ernst Stoller, an officer who was born and brought up in England, to substitute for one of the dead men. Meanwhile, Monique is becoming increasingly disillusioned with her life at the Candide and relationship with Albert, and finds herself letting her guard down when she becomes involved with Stoller as part of her Lifeline work.

Lifeline once again must tread carefully when one of their safehouses is blown. Meanwhile, Van Reijn, a minister in the puppet Belgian government, wants to defect to England and contacts Curtis. Curtis tries to persuade Lisa to let Van Reijn be sent down the line, citing the propaganda value of such a defection. However, matters become complicated, as Lifeline has once more been secretly infiltrated by a German officer.

The citizens of Brussels are ordered to celebrate the Fuhrer’s impending birthday, and Albert and the others are suspected of being collaborators when they must hang a Nazi flag outside the Candide. Meanwhile, a man claiming to be an RAF evader approaches Catherine Bidout, an elderly English woman who has not yet been interned by the Germans. Rumours of a British woman in Brussels hiding an evader reaches the Candide and Albert decides to investigate. Madame Bidout is uneasy about the risks she is taking and contacts an old friend – Brandt – for help. However, the evader is a plant placed by Kessler to entrap Lifeline and now Brandt is also in danger.

As Easter approaches, a monk from a local order, Father Girard, visits the Candide to hear Andree’s confession and also overhears a conversation about an evader. He offers his assistance to Lisa, who gratefully agrees that the airman can be hidden in the abbey over Easter. At the abbey, the head of the order, Father Pierre, is happy to help Lifeline, however another of the brothers, Brother Anselm, strongly believes that the order should not endanger itself by getting involved with the resistance and misguidedly contacts Kessler.

Following up a lead in their continuing investigations to crack the evasion lines, Kessler and Brandt examine the details of a recent murder case in France with Curtis, in his cover as a travelling salesman, as their prime suspect. Believing Curtis to be part of the resistance, Brandt and Kessler visit the Candide and take him away for questioning. With their efforts to help evaders becoming more and more dangerous and difficult, Albert and Lisa fear the worst, as the Germans strike right at the heart of Lifeline.

Some months have passed, and Albert and Monique are preparing for the grand opening of the Restaurant Candide on Brussels’ Grand Place, financed by London to allow Lifeline to continue its activities. Lifeline also recruits a new forger, Max Brocard. Alain brings a British agent to the Candide with an urgent request from London: a Brigadier General with information on a major Allied offensive in North Africa (named in the programme as Operation Torch) has been shot down on his way to a top secret meeting and must be rescued at all odds. Natalie has already discovered the Brigadier and prepares to take him down the line. However, disaster strikes when a German soldier is murdered, and in retaliation, Kessler orders that twenty people are to be held hostage and shot if the killer doesn’t give himself up. The Brigadier is one of the hostages.

Dr. Keldermans visits the Candide with tragic news, and Albert must take decisive action if Lifeline is to continue. Meanwhile, Natalie has the difficult task of taking two eccentric Russian evaders down the line. Brandt dines with a high-ranking military acquaintance, Oberst von Elmendorff, who tries to recruit him in a military plot to assassinate Hitler, while Kessler invites Albert to a strange garden party.

Lifeline must work overtime when a heavy storm downs several British planes on their way back from a bombing raid on Berlin. Natalie comes into contact with Major Nicholas Bradley, a British officer on a special assignment, who is uncooperative and highly critical of Lifeline’s security when he follows her all the way back to the Candide. It seems that Bradley’s concern is well founded, as an undercover Gestapo man is also tailing Natalie.

Natalie’s new boyfriend, Francois, is keen to become involved in Lifeline, but Albert has doubts. Paul Vercors, the fanatical leader of a Communist cell in Brussels, wants to take over Lifeline, and tries to enlist the assistance of a colleague – Max. Meanwhile, the Communists continue their resistance activities and plant explosives on a railway line to derail a train. Unfortunately, the train’s passengers include Natalie and Francois, escorting evaders down the line.

Brandt receives a visit from his wife Erika, who asks him to let her and their children come to live with him in Brussels as it is becoming too dangerous to carry on living in Berlin with the increasing British air raids. Brandt tells Erika that his work in Brussels may soon be over, as he has successfully infiltrated Lifeline with an undercover agent. Word of this reaches the Candide, and Albert must face the grim possibility of killing all evaders currently in hiding to preserve the safety of the line. However, Max and Dr. Keldermans have an alternative plan to discover the identity of the infiltrator, but the scheme requires the assistance of Natalie’s uncle, Hans Van Broecken, who hates Albert and will not help Lifeline.

Kessler is enjoying his developing relationship with Madeleine, but begins to feel that his feelings for her may cloud his judgement while Brandt is once more approached about joining the military plot to assassinate Hitler. The British co-ordinate a massive air raid attack on Berlin, but bombs are dropped on civilian targets in error – with tragic repercussions for Brandt. Monique and Alain try but fail to help a bomb aimer shot down on his way back to England. Having captured the man, Brandt finds himself face to face with the enemy and this time, it’s personal.

A British Wing Commander is badly burned after he has been shot down over Belgium. London wants him back urgently, but his facial injuries make it difficult for him to go down the line. He manages to get to Paris with Natalie as his guide, but Monique decides that if he is to avoid detection he must disguise himself as a woman for the rest of the journey. Meanwhile, Madeleine is ostracised for her relationship with Kessler, but finds a new friend in Monique. The Candide and its staff are targeted by terrorists for being collaborators, and Natalie and Max fear the worst when the Gestapo arrest Albert.

Christmas is approaching, and Lifeline finds itself having to play Saint Nicholas when a wounded Group Captain is discovered by a group of abandoned children fending for themselves, who are prepared to sell the officer in turn for food and presents. However, Albert fears that it could all be an elaborate trap set up by the Germans. Meanwhile, Brandt finds his loyalties to Germany divided after recent events and confides his worries to Kessler.

The Germans are out to capture a group of evaders, believing them to be responsible for the murder of an officer. One pair of evaders manages to contact Lifeline, while the other two throw their lot in with the Communists. Francois is determined to prove his worth as a Lifeline guide to Albert and Natalie, and decides to co-ordinate the journey down the line for all the evaders, approaching the Communists to take responsibility for the airmen against Albert’s advice. Whilst visiting the Communists’ hideout, Francois meets Max, whose true loyalties have begun to emerge in force.

Kessler tries to help Brandt as he tries to cope by heavy drinking and flirting with Monique at the Candide. Alain is forced to hide two evaders on his farm when a neighbour finds a wounded German fighter pilot on his property. It transpires that the pilot is a decorated flying ace and his plane is on the German secret list, and London asks for Lifeline’s assistance in getting him to England as a prisoner of war. Meanwhile, as Max prepares to bring his plans for Lifeline into fruition, Natalie demands that Albert take action.