This week, Blue Watch are called to a fire at Station Officer Mick Callaghan's house. His wife, Shauna, who has been suffering even more violent mood swings, is trapped inside with their young son, Liam.
What nobody has yet diagnosed is that Shauna is actually suffering from manic depression. Even Mick has been ignoring her cries for help.
But how does an actor portray such an illness? Liza Walker, who plays Shauna, says she drew on the memory of her own lowest moment to understand her character's illness.
Four years ago Liza's close friend, model and actress Kadamba Simmons, was murdered by a former Israeli soldier Yaniv Malka, whom she met in India. Malka was given life imprisonment at London's Old Bailey in March 1999.
'I was devastated,' says Liza. 'The world seemed a very unhappy place.'
A four - month spell of travelling in India got Liza back on track.
'Following in Kadamba's footsteps helped me to make peace with myself. In a funny kind of way, her death eventually made me feel more positive about life. But at the time I was deeply depressed.'
although her depression wasn't a chronic illness like Shauna's, Liza's grief gave her a little insight into how someone like Shauna could be feeling.
To play the role, she sought expert guidance on manic depression. She didn't receive any - and that left her fuming.
'I got absolutely nowhere with specialist organisations. I told them I was researching a role that would be seen by millions and that I wanted to get it right. But still nothing.
'To be honest, I was disgusted. Maybe they were suspicious of my motives. But I hope they do more for people who are actually suffering from manic depression.'
Liza eventually contacted a friend, who suffers from the illness, for help.
Happily, today Liza has plenty to smile about. She marries her musician fianceé Donnie Hardwidge this September and will become stepmum to four - year - old Jackson.
'I get on really well with Jackson - we're best friends. He even calls me 'My Liza', which I love!'