The third element in Wendy and David Johnstone's marriage was not another person but the insidious condition known as obsessive compulsive disorder.
For 20 years they were slaves to more and more bizarre rituals as Wendy's condition spiralled out of control.
Could you live with a man who constantly put himself in danger? Robin and Heather Horsfall talk to Ros Taylor about how Robin's job as a mercenary in the world's hotspots put a constant strain on their marriage.
Today she meets the Cannings, who are fighting Angela Canning's conviction for killing her child.
Could your marriage survive the SAS treatment? Ros Taylor finds how one couple have stuck together through the tough times.
Pauline and Mick Holcroft tell Ros Taylor how their relationship has survived the drug addiction and death of their daughter.