“Ruling side by side, we must do so in harmony — not through a treaty drafted by men lesser than ourselves,” says Saoirse Ronan in the first trailer for “Mary Queen of Scots.” The statement is nothing more than unfortunate foreshadowing of what is to come.

“Mary Queen of Scots” centers on the relationship between Mary Stuart (Ronan) and her cousin and the Queen of England Elizabeth I, played by Margot Robbie. After Mary, Queen of France, is widowed at 18, she returns to Scotland to reclaim her homeland, which is now under the English throne. By asserting her place among the sovereign, Mary threatens Elizabeth’s crown, diplomacy between England and Scotland, and her relationship with her cousin. As the official press release describes, “Rivals in power and in love, and female regents in a masculine world, the two must decide how to play the game of marriage versus independence.”

Beau Willimon’s screenplay is based on John Guy’s biography “Queen of Scots: The True Life of Mary Stuart.”  Debra Hayward (“Harlots”) is among the film’s producers.

Director Josie Rourke comes with a background predominantly in British theater. Her past productions include “Much Ado About Nothing,” “The Vote, “Les Liaisons Dangereuses,” and “Saint Joan.” “Mary Queen of Scots” marks her feature directorial debut.

Since their Oscar-nominated roles in “Lady Bird” and “I, Tonya,”  both Ronan and Robbie have been keeping busy. An adaptation of Ian McEwan’s “On Chesil Beach” starring Ronan recently hit theaters and is set for and DVD and VOD release July 17. Her next projects include refugee drama“Sweetness in the Belly” and “Lady Bird” helmer Greta Gerwig’s rumored “Little Women.”

Robbie is signed on for a Harley Quinn spinoff, trapeze artist pic “Queen of the Air,” and “Beautiful Things,” a thriller about a mother and son trapped in a zoo. She’s also set to play the titular role in “Marian,” a spin on Robin Hood folklore that puts the maiden at the forefront.

“Mary Queen of Scots” hits select theaters December 7.