This quest immediately follows up the quest 'Natah' so doing some minor spoilering of that one to catch people up to speed. Long and short- Tyl Regor, the Grineer's top egghead, accidentally awakens the Sentient known as Hunhow. During this quest you learn that Lotus (aka Natah) is a Sentient herself, and in fact was one of the reasons the Tenno wrecked the Orokin in ages past. However, she grew needy for the family and adopted the Tenno instead, hiding them in 'the second dream' until such time they needed to awaken. Exposition as you defend explosions to lock Hunhow back up.
Fast forward to 'The Second Dream' itself. Hunhow has recruited the Stalker (who now has some fresh new cosmetics thanks to sentient remains). With the assistance of a recently-cured Alad V, Lotus sends you to try and track down what the Stalker may be up to, only to accidentally point him right where he wants to be- the moon, which the Lotus has hidden into the void after all these years, rumored destroyed during the Old War. Lotus and you do your damnest to try and keep the moon locked in the Void (so that the Sentient units sent by Hunhow can't help, as they do not take the Void well), but all is for naught, and you are sent to the Reservoir to stop Stalker from destroying it.
And thus, we find out the biggest plot twist. All this time, the Operator (aka the figure behind the Tenno 'shell' you've spent all this time as) has been in the Reservoir, safely hidden by the prying eyes of all the forces. The end of the quest has you bringing the Operator back to the landing craft and placing them in a Somatic Link located on the craft, all the while fighting with the Stalker trying to push you back. Then you get to actually have a chat with the Lotus face to face, customize how you want to look in your little reclining spot, and unlock the Focus system, which is accessed from that spot.
There's a lot of nice little touches too, like when you travel along the moon and hear echos of the Orokin, speaking about the Tenno and some of the other events. Definitely something I suggest to play for yourself, though it takes quite a while to get to Uranus to unlock Natah and then play through it as well. Also the new Sentient are OP as fuck, you only fight one at a time but they will suck you dry by the end of it ^^;