Fans still have a month of waiting left to do before "Grey's Anatomy" season 12 returns to TV screens, but the anticipation is already starting to build. One story line that has had people wondering since the midseason finale in November is the bad blood that exists between Owen Hunt and new resident Nathan Riggs.

Nathan (Martin Henderson) arrived in Grey Sloan Memorial Hospital as the new cardio attending in episode 7 and immediately caught Owen's (Kevin McKidd) ire. The trauma surgeon's mysterious anger at the new resident causes strain among the other doctors and tests his relationship with Amelia (Caterina Scorsone).

Things finally came to a head in the show's midseason finale, "Things We Lost in the Fire." In the episode, Owen revealed to Meredith (Ellen Pompeo) that he had a sister. While surprised with the revelation, she promised to keep his secret. This in turn led to conflict between Meredith and her sister Amelia, who became angry that her boyfriend would confide in someone else.

While no one seems to know about Owen's past and his unknown sister, the odds are good that Owen's ex-wife, Cristina Yang (Sandra Oh) knows about her. McKidd admits as much in an interview he gave to The Hollywood Reporter. According to the actor, his gut tells him that Christina might be "the only one who knew but was sworn to secrecy." Since the two characters had a long and quite complex relationship, McKidd says that part of his life would have come up.

The 42-year-old actor also shared that Nathan and Owen's sister were definitely romantically involved but something unforgiveable happened.

McKidd assured fans that they will definitely learn the reason for Owen's anger and secrecy. But when they'll find out is the big question right now. It's possible that it will be revealed in the series' midseason premiere, "The Sound of Silence." In an Entertainment Weekly report, show creator Shonda Rhimes describes the episode as "extraordinary and very, very powerful, and directed by Denzel Washington."

"Grey's Anatomy" season 12 will return to ABC on Feb. 11.