Call the Midwife features powerful domestic abuse storyline

When Sister Veronica (Rebecca Gethings) first meets Sandy (Rose Riley) and Joe Talbot (Mark Strepan), they have just tied the knot and appear blissfully happy, basking in the glow of their love. But when the health visitor returns to the couple's home the next morning to pick up where she left off, it's evident that all is not well.

Joe has been subjecting Sandy to emotional, physical and sexual abuse, a harrowing reality that she has desperately tried to keep hidden.

"When nobody knew, it didn't matter," she says. "I didn't have to be ashamed."

A short time after confiding in Sister Veronica, Sandy flees her home in the middle of the night with her two children and is discovered by Fred in the alleyway next to the shop. A short time later, she's put up in a women's shelter.

But in her haste to escape her husband, Sandy didn't pack her son's favourite toy and heads back to the flat with Sister Veronica to collect it, only to discover that Joe has changed the locks.

Unwilling to suffer any further injustice, Sandy takes the matter to the police. After the officer informs her that he can't take action as it's a "civil" case, she decides to report the sexual violence she's suffered.

But the detective has some crushing news for her: rape within marriage isn't a crime.

"The offence does not exist," he says to a stunned Sandy and Sister Veronica.

When did marital rape become illegal?

In the 1600s, English jurist Sir Matthew Hale wrote the following in his common-law treatise: "For by their mutual matrimonial consent and contract, the wife hath given herself up in this kind unto the husband which she cannot retract."

Barrister John Frederick Archbold supported that assertion in 1822 in his own legal document: a husband "cannot be guilty of a rape upon his wife".

But there was a seismic change in 1991.

In a case titled R v R, a man who was convicted of attempting to rape his wife challenged the decision on the grounds of the marital rape exemption. But the House of Lords reviewed the case and overturned the original ruling.

"The common law is, however, capable of evolving in the light of changing social, economic and cultural developments... one of the most important changes is that marriage is in modern times regarded as a partnership of equals, and no longer one in which the wife must be the subservient chattel of the husband.

"Hale's proposition involves that by marriage, a wife gives her irrevocable consent to sexual intercourse with her husband under all circumstances and irrespective of the state of her health or how she happens to be feeling at the time. In modern times any reasonable person must regard that conception as quite unacceptable."

But season 12 of Call the Midwife is set in 1968, with Matthew acknowledging that the "legalities are pretty grim" for women like Sandy.

Given that her husband hasn't "deserted" her or "committed adultery", she will have to "try and prove cruelty" in court – and staggeringly, rape cannot be used as a defence.

"Legally, rape is only grounds for divorce if the husband rapes another woman, not his wife," explains Matthew. "Also, because you've been married for under three years, you will have to prove exceptional hardship or depravity."

But Sandy is not deterred by the odds stacked against her and vows to expose Joe's brutality. She's entitled to legal aid and Matthew offers to help her find a solicitor, and she also has a signed letter from Sister Veronica in support of dissolving her marriage.

"I wanted to say thank you," she says to the health visitor. "Because when I saw you shouting, I knew I could shout too."

For information and support, visit the NHS website or Rape Crisis England & Wales.

Call the Midwife airs on Sundays at 8pm on BBC One. Check out more of our Drama coverage or visit our TV Guide and Streaming Guide to see what's on tonight.