British actor Jude Law, shown in September, will play Dr. Watson in a Sherlock Holmes film to be directed by Guy Ritchie. (Musadeq Sadeq/Associated Press)

Jude Law will be Dr. Watson, sidekick to Robert Downey Jr.'s Sherlock Holmes, in a new version of a film about the London detective.

Law was announced as Watson on Thursday at a news conference in which director Guy Ritchie revealed the Sherlock Holmes movie will be shot in London over a three-month period.

Locations include St. Paul's Cathedral and Freemason's Hall.

Ritchie's script is based on the graphic Lionel Wigram novels, which take Arthur Conan Doyle's stories in a more adventurous direction.

Holmes, while still eccentric, is a sword fighter and man of action, as well as the deep thinker and scholar of forensic clues he has always been.

"I came to the character through the books when I was young, and what the film will reflect is the intelligence on the page and also the action," said Ritchie, director of RocknRolla and Lock, Stock and Two Smoking Barrels.

"There's quite a lot of intense action sequences in the stories, sometimes that hasn't been reflected in the movies. It's been a relatively long time since there's been a film version that people embraced. I want to make a very contemporary film as far as the tone and texture."

Ritchie described the film as an action picture, and said it would be a departure from his usual style.

Iron Man star Downey Jr said he was studying this new version of the detective.

"The more I look into the books, the more fantastic it becomes. Holmes is such a weirdo," he said.

The film also stars Canadian actress Rachel McAdams as Holmes's love interest, Irene Adler, Mark Strong as villain Blackwood, and Kelly Reilly as Watson's love interest Mary.

The film will come out in 2009, but ahead of a competing film about Sherlock Holmes that will star Sacha Baron Cohen.