River Song is not new to all diehard Whovians. But those who have just been onboard with the science fiction show and who only know the Twelfth Doctor may have a few questions about the returning "Doctor Who" companion. So right before diving into the Christmas special, perhaps it's best to ask, "Who is River Song?"

River Song is returning to the "Doctor Who" Christmas Special 2015 and will be played by the same actress who portrayed the character since her debut in 2008, Alex Kingston. While those who have been hooked on the science fiction drama since at least season 4 will be familiar with River, newer fans (or those who have only tuned in since season 8) will have no background whatsoever about her even if she has had a long history in the time of The Doctor.

Besides the fact that she will have an appearance in the special "Doctor Who" episode come Christmas Day, she also, at one point, became the wife of the Eleventh Doctor, played by Matt Smith.

Den of Geek compiled River Song's chronological timeline, as her story was told throughout the series in a jarring timeline, jumping from adulthood to childhood then back again, to explain how important the returning character is to the Doctor's life.

Her childhood was glimpsed in the 2010 episode, "The Eleventh Hour," which explained why River Song is tied to the Doctor whether she likes it or not. Though technically not a Time Lord, she was born with the characteristics of one, no thanks to the tinkering of her parents Amy Pond (Karen Gillan) and Rory Williams (Arthur Darvill), who were once companions of the Eleventh Doctor. River Song was born as Melody Pond and regenerated as her current reincarnation after her death.

The article then goes on to talk about her rebellious adulthood, her life as an archaeology student at the Luna University (and her eventual marriage to the Time Lord), and her afterlife. It is a long but very interesting read, especially if one wishes to fully know the character they are about to meet in the special, "The Husbands of River Song."

In a nutshell, though, River is a very important character for the Doctor and her return does beg a special episode.

But while River's last appearance on the show was a bit grim, her re-emergence is going to be a fun one. It will be her first encounter with the Twelfth Doctor and it will be interesting to see how she goes on to discover that the man she picked up and now traveling with was her one-time husband.

"It's River Song, back in the Doctor's life. He probably thought he'd never see her again. So, when she turns up, it's quite a shock for him. It's a very pleasant shock for him, because he loves his River," executive producer Steven Moffat told Entertainment Weekly of River's return.

Get to know more of River Song when she reappears in the Doctor's life in the "Doctor Who" Christmas Special 2015, "The Husbands of River Song," airing on BBC (UK) at 5:15 p.m. and BBC America at 9 p.m.