Warner Bros.

It's elementary, dear moviegoers — Helena Bonham Carter is joining the cast of the Sherlock Holmes spinoff movie "Enola Holmes."

She'll star opposite Millie Bobby Brown, who is playing the titular character, the much younger sister to Sherlock and Mycroft Holmes. Carter will play Enola's mother.

The "Enola Holmes Mysteries" YA book series began in 2006 with "The Case of the Missing Marquess" and continued for five more novels. Enola turns out to be a highly capable detective in her own right.

Carter starred in last year's "Ocean's Eight" and can next be seen as Princess Margaret in Netflix's "The Crown" (taking over for Vanessa Kirby).