Where do we begin on the Second Dream quest. The crux of the story is that The Stalker's pursuit of Tenno has led him to ally with the recently awakened sentient being Hunhow. Hunhow knows the location of The Lotus, and where she has hid the Reservoir, that we are told is the Tenno's hidden weakness. After a few missions in attempt to find the Reservoir before The Stalker, It is then revealed that the Reservoir is actually hidden on Earth's Moon, long thought to have been destroyed during The Old War. The Moon has been kept inside the Void all this time. Furthermore, the Reservoir has been the true resting place of you, The Operator. I and a lot of other Warframe players always suspected that the Warframe was nothing but an AI. The fact that you can possess multiple Warframes yet The Lotus always referred to the Player as "Tenno". I thought the reveal of the player and the Warframe being separate entities was well done. The mission leading up to this reveal has to be done Solo and the Sentient Fighters and Mages (on the Moon) is quite challenging to defeat by yourself. I found out after completeing the quest that defeating them is actually totally optional. The challenge of defeating them was I guess rewarded with the cathartic moment of when the Operator is able to melt several Sentients with his or her "Focus" (It's a laser hand... just saying) while being carried in the arms of the operator's Warframe.
Which leads me to the climatic scene back on the Tenno Ship, Liset where once again (somehow) The Stalker makes it on board the Orbiter, Even the Lotus exclaims, "Stalker? How?!". In the last cut-scene with Stalker he stabs "War" the sword he has been wielding into the operator's Warframe. We get dialogue from Hunhow talking to Stalker who is now chocking the Operator. This is when the Warframe miraculously comes to life and breaks the sword in its chest into half. Which somehow severed the link between Stalker and Hunhow. Which leads to the speculation if the Warframe come to life on its own? Up to this point Hunhow had been monologing about how the Warframe are just metal puppets of the Tenno. How the only way for Stalker to end his despair is to kill the Operator. Perhaps The Stalker himself is just a Warframe, and was control by a Tenno at one point, and realized it was being used for War, and wanted revenge on the Tenno. Nevertheless The Stalker's true backstory still remains in question. After the sword is destroyed, the Stalker disappears the blinding light as he leaves, knocks the Tenno unconscious.
The final scene, Lotus mysteriously appears (where were you all this time?!) and carries the Tenno to the Somatic Link a chamber in the Oribiter where just prior the Player was fighting The Stalker. It is at this point where we are able to customize our true-self, I made myself look like an anime mobile suit pilot of course. If it is not blatantly obvious, Lotus spells it out that this is our trueself speaking directly to the newly customized character. As a player who had been playing and following this game's updates for more than a year and half now this moment made me scream like a little fanboy inside. (Digital Extremes pulled a reverse MGSV on us!)
Lotus gives you some backstory into your past. The game will then allow you choose between one of the 5 Tenno "schools" of philosophy, and like a sort of personality test, based on some of the dialogue choices you make at this point it will recommend which School to choose. Although I regret picking Vazarin, do not pick waves it is by far the least useful School out of the 5) The school will unlock essentially a the Focus system, Tenno Skill true. A 5th ability.