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.

The safety of Lifeline is at risk when Albert deduces that the Candide is being watched by a Gestapo spy, while Lisa continues to resent Curtis’ presence and his domineering take-charge manner. A wounded RAF evader, Finch, finds sanctuary with Hans Van Broecken, a Dutchman who lives on a barge with his wife Lena. Finch plans to desert the army and escape to Switzerland and finds a kindred spirit in Hans, who is in fact German and deserted during the First World War. Hans plans to take Finch to Switzerland himself on the barge, but Lifeline becomes involved when Lena receives a visit from her niece – Natalie. However, helping evaders escape to comfortable Swiss internment is not Lifeline’s purpose, and how can Finch be involved with Lifeline with the Candide under German surveillance?

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.

In the aftermath of the Romsey business, a German newspaper publishes British military secrets. Curtis decides that the traitor must be found and dealt with. He travels to France and retraces Romsey’s steps. Meanwhile, Albert is still suspicious of Curtis and decides that once he is away from Brussels he will reveal his true colours. Monique follows Curtis to France to watch him and finally prove whether he is trustworthy or not, while Curtis’ investigations lead him to the Neville residence.

When Lisa Colbert is arrested whilst meeting a contact near Paris, the whole future of Lifeline is put at risk. This also leads to her uncle being arrested and interrogated by Kessler. Whilst in prison, Lisa must decide whether she can trust her fellow inmates who claim to know who she is and how they can help her escape. These events will lead to tragedy, both personal and for Lifeline.

With so many evaders, Lifeline’s resources are once more stretched to the limit and they must watch their backs after recent events. Jacques and Natalie are dispatched to check out a young South African evader, Noel Uys, and have to consider eliminating him when his answers to their questions fail to satisfy either them or London. In desperation to survive, Uys overpowers Jacques and armed with a shotgun, takes Natalie as his hostage.

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.