Robert Downey Jr disguised as a woman with Jude Law in Sherlock Holmes: A Game of Shadows and (above) Jared Harris as James Moriarty

Moriarty is back. After a too-brief cameo in the 2009 hit film Sherlock Holmes, the detective’s most notorious nemesis becomes the focus of the sequel, scheduled for release on December 16.

Even with Mark Strong playing the bad guy as the serial killer Lord Blackwood — and an impressive $209 million haul — director Guy Ritchie felt something was missing the first time. Although he’s reluctant to do sequels, he was quick to reteam for Sherlock Holmes: A Game of Shadows.

“I can’t help myself,” Ritchie says. “They wanted this to be a franchise, which, in my mind, needs at least two movies. But I really liked working with the team on the first movie, so this was an easy choice.”

So was the primary villain for A Game of Shadows. The cast will reunite Robert Downey Jr as Holmes, Jude Law as Dr Watson and Jared Harris as the inscrutable James Moriarty.

The movie, which also sees the return of Rachel McAdams, follows Holmes as he and Watson trace Moriarty’s murderous trail with the help of Holmes’s older brother, Mycroft (Stephen Fry), and a Gypsy girl, Sim (Noomi Rapace). The film shares elements from Sir Arthur Conan Doyle’s 1893 short story The Final Problem, which first appeared in Strand magazine and introduced Moriarty.

Ritchie, director of Lock, Stock & Two Smoking Barrels and Snatch, says he was taken aback by the success of the first Holmes film and wasn’t sure how to repeat it.

But a good baddie never hurts.

“Moriarty is one of the most famous villains of all time,” he says. “But we didn’t want to make him a conventional character. The challenge was to still make him modern, believable.”

Conventional, Harris isn’t. Recently known as the nerdy British pencil-pusher from Mad Men, Harris “brings a depth, a realism, that everyone was really excited about,” Ritchie says. “He’s very relatable.”

Even if Holmes’s appeal remains something of a puzzle to Ritchie.

“I don’t know why he’s been so popular all these years,” Ritchie says. “Maybe it’s because he was such a pioneer. He was the first famous reporter. The famous investigator with flaws in his character.

“Yet he’s still this perennial winner. How could you not like that?”