River Song has not appeared on Doctor Who since 2013's "The Name Of The Doctor". the episode leading up to the shows big 50th anniversary episode. There's been speculation over the last two years as to wither she'd return or not and now, finally, she is.
Kingston seems happy to be back as Song,
“To be honest, I did not know whether River would ever return to the show, but here she is, back with the Doctor for the Christmas special. Steven Moffat is on glittering form, giving us an episode filled with humour and surprise guest castings. I met Peter for the first time at Monday’s read through, we had a laugh, and I am now excited and ready to start filming with him and the Doctor Who team. Christmas in September? Why not!”
Lead Writer, Steven Moffat, seems pleased with bringing back the character too, but isn't giving too much away as to how we'll see her this time and what she'll be like,
“Another Christmas, another special for Doctor Who - and what could be more special than the return of Alex Kingston as Professor River Song? The last time the Doctor saw her she was a ghost. The first time he met her, she died. So how can he be seeing her again? As ever, with the most complicated relationship in the universe, it’s a matter of time...”
I'm totally happy and excited with this return as River Song is and always will be an amazingly strong character in the NuWho universe. I wonder what's up Moffat's sleeve this time round with the character? I guess only time will tell.
A phone in Stormcage Containment Facility in 5145 rings. A guard picks it up and says that there was no Doctor there. At that point River Song demands to speak to the person on the other side of the call. River realises he is new, and kisses him, using a hallucinogen in her lipstick to make him delirious, and she escapes. Then we head over to The Royal Collection where River Song walks among the paintings in the Royal Collection gallery and finds the one she is looking for. She trips one of the alarms. Liz 10 then appears with a gun aimed at her. River tells her that this is for The Doctor, and Liz recognizes the name. River shows her the painting of the exploding TARDIS. River then heads to The Maldovarium where she talks to a salesman at a bar, asking for a vortex manipulator from a Time Agent in return for a device for deactivating micro- explosives similar to those she had planted in the salesman's drink. 
The Doctor and Amy then head back into the TARDIS and end up landing in Roman Britain in 2nd Century AD. The guards that greet them when landing, takes them to see Cleopatra, who turns out to River Song in disguise. Song shows The Doctor the painting of the exploding TARDIS. River's previous warning that the Pandorica would open also concerns him, and the Doctor decides to look for it. He looks at the Roman maps, since if someone had built such a device, they would want to remember where they left it. The three of them leave the Roman camp and travel by horse to Stonehenge to see if they can find the Pandorica. The Doctor uses his sonic screwdriver and it picks up some activity below Stonehenge. They find an enterence to below. There they find Pandorica. The Doctor then relises that the Pandorica is not a fairytale or a myth after all. IT'S REAL! Amy says that the real box is so similar to the one in the story book of the same name. The one she read as a child.
Back at Stonehenge, a Cyber arm on the floor of the chamber has reactivated, sending bolts of energy from it's gun around the room. The Doctor and Amy run for cover behind the Pandorica. The Doctor says to Amy that he has a plan. So his plan turns out to be him running out from behind cover and shout, "Look at me, I'm a target!". Classic "Doctor Who" moment. Hillarious! He manages to get close to the arm. He tries to pick it up, but the electronic grip function on the arm incapacitates the Doctor. That electric shock thing that the Cybermen do to hurt people. By now, the head has also worked it's way down using tentacle-like wires from the neck section and a sleeper dart from one of the pipes hits Amy. The head opens. The Cyberman is a type-2 upgrade, with the parts grafted directly onto the body. The skull of the previous occupant is ejected, and the body attempts to assimilate Amy. After getting attacked by the Cyberman, who is missing it's head and arm, Amy takes cover in a side room, but before the Cyberman can get to her, it is killed by the Roman Rory. But Amy doesn't recongise him. 
The Doctor wakes up and notices Rory, hardly reacting, and begins his speculations again. However, after a few minutes, he notices and they go through an awkward phase. This was very funny. The Doctor decides to address the fleet of ships in orbit, while River goes to fetch the TARDIS for some technology. The Doctor gives a speech on how he had defeated most of those aliens at some point, and the fleet falls back, giving them about half an hour to think things through. 
When River gets into the TARDIS, she attempts to go back to Stonehenge, but instead is thrown to 26th June 2010, the date that appeared on the base code of the universe and as such when it would end. She leaves the TARDIS just before a warning is broadcast on the speakers. The speakers were saying, "Silence will fall", and then the screen at the centre console cracks. There are scorch marks on the ground, and River enters the house nearby, which has been broken into. By some of the things she finds, she notices that the house is Amy's house. River goes up into Amy's room and find a book on Roman Britain and Pandora's Box. These are the exact things that is happening right now, where The Doctor is. 
She tries to return to inform The Doctor of her findings, but is instead thrown uncontrollably around the Time Vortex. She phones the Doctor to get help, who tells her to shut off the engines once he learns the date she is at, "26th June 2010". The base code of the universe the Doctor had seen the last time he met River was 26062010. She can't deactivate the engines, so he tells her to shut them off automatically by landing and leaving the TARDIS. She lands, but the doors are locked. 
The Cybermen arrive and their Cyber-Leader states that all evidence correlates, The Doctor is to blame. Silurians, Sontarans, Judoon and Sycorax arrive and The Doctor realizes that they have formed an alliance to save the universe from him. The alliance had built a perfect trap, using Amy's memories as building blocks. Very clever idea. 
The alliance believe that The Doctors TARDIS exploding will cause the end of the universe. The Auton Rory is still fighting, but he ends up shooting Amy. That was a really imotional scene. She then dies in his arms. AWWWWW! The Pandorica finally opens and inside is a holding cell, designed for a humanoid. A prison for The Doctor. To keep him from damaging the universe, even though he's the one who protects it from the villains. So, the Daleks say that it is for the Doctor. The Doctor tries to protest, saying that it is the TARDIS that is the threat, not him, but the Daleks state that he is the only person who can fly it, since they are unaware of River's presence and have him strapped into the holding cell. Watching Matt Smith fighting for his life and begging them not to lock him up was again, so imotional and such a powerfull scene. It was like watching David Tennent fighting for his life when he was The Doctor. With the Doctor held firmly in place, the Pandorica closes. 
River manages to open the TARDIS door, but in front of her is a wall of stone. She says that she is sorry, and the TARDIS console explodes. There are thousands of explosions across the sky, and Earth is left in the middle of a void. Silence falls.