Long-time collaborators Pedro Almodovar and Penelope Cruz took an unexpectedly political turn for the opening film at Venice on Wednesday, with a drama touching on the missing victims of Spain's civil war.

Exploring blood ties and buried history, Parallel Mothers marks a departure into dark historical territory for the director, while still focusing on the themes of motherhood and female relationships that have been central to many of his films. 

Cruz described Almodovar as "my safety net" in a press conference ahead of their red-carpet appearance in Venice. 

"He can ask me to do something that can really scare me but I know he will be there waiting to sustain me," she said, adding that she was grateful to the director for giving her "so many different, challenging characters".

Cruz has appeared in seven of Almodovar's movies, including All About My Mother and Volver.

Their latest again features strong, independent women, but also weaves in a storyline about Spain's "disappeared", the tens of thousands who remain buried in unmarked graves from the civil war of the 1930s.

"Spain has an enormous moral debt to the families of the disappeared," Almodovar told reporters.

"After 85 years, until we pay this debt with the disappeared, there can be no definitive closure with everything that happened in the civil war," he added. 

More than 100,000 victims of the war and its aftermath under Francisco Franco's dictatorship are still missing, according to historians and victims groups. 

The film follows two single mothers of different ages, played by Cruz and newcomer Milena Smit, who bond while sharing a room in a maternity ward. 

While Cruz's character deals with a mystery surrounding her baby's identity, she is also involved in efforts to unearth a suspected wartime burial site in her hometown.

It was perhaps a surprisingly serious turn for a film that initially made headlines when its poster was briefly banned by Instagram this summer for featuring a lactating nipple. 

There were still plenty of Almodovar's usual flourishes, from the backgrounds full of outrageous colours, to a T-shirt sported by Cruz reading: "We should all be feminists."