From Halopedia, the Halo wiki
Key is the ninth episode of Spartan Ops. It was released on February 11, 2013. In this episode, ONI has ordered Doctor Halsey assassinated, and Sarah Palmer is slated to pull the trigger. Lasky, in an effort to save Halsey's life, sends Fireteam Majestic to stop Palmer. What they discover may be the key to the future of humanity.
Palmer receives her M6H pistols from a nearby technician.
Palmer receives her helmet from the technician, and turns to face Lasky.
The two begin leaving the armory to head to the hangar.
Palmer stops and stares at Lasky.
Palmer continues walking away, stopping when she reaches her deployment panel.
Palmer puts on her helmet. The floor lowers to deploy her to Requiem's surface.
Halsey stares at her in awe.
Cut to many patrolling Sangheili. Cut to Sarah Palmer, who pulls out a combat knife, sneaks up behind an Elite, and stabs it in the neck. The Elite gurgles and dies, and Palmer sets down its body before carefully sneaking ahead.
Cut to "Sniper Alley," where many Elites patrol its entrance. Gabriel Thorne sneaks by while cloaking, but one Elite senses his presence and looks around for him. Thorne uncloaks behind him, and the Elite spots him and whips out his energy sword, as do the others.
On the ground, DeMarco attacks the Elites with his energy sword. He assassinates one, swings the legs out from another, and overpowers the clash of a third. As he turns to defend behind him, his back bumps into the decloaking Thorne, who has been picking off Elites with his assault rifle.
Thorne nods and covers for DeMarco with a few more shots. He then turns and dashes inside the base, recloaking as he does so.
Cut to Jul 'Mdama in front of the Librarian's shrine. He approaches Halsey's unconscious body, which is floating inside the beam.
Cut to the Librarian inside. She holds up two halves of a small thin device.
The Librarian connects the two pieces together. Holograms of star systems burst from it and surround the two women. Halsey examines them in fascination.
The Librarian separates the pieces, and the holograms disappear. She holds out the pieces to Halsey.
Halsey cautiously takes the pieces from her. With a flash, Halsey is flung out of the artifact, still clutching the keys. 'Mdama catches her, notices a piece, and takes it from her.
'Mdama drops Halsey and examines the half-key, unaware that Halsey has the other.
One of 'Mdama's guards suddenly screams and is thrown to the floor. Thorne uncloaks and pulls out his assault rifle.
'Mdama grabs Halsey and holds her in front of him.
The other guard swings its sword. Thorne ducks, hits its head, then fires an assault rifle burst into its chest.
Halsey tosses her half of the Janus Key to him. Thorne catches it, and examines it. He looks up upon hearing a gunshot, and sees an Elite about to attack him fall dead from Palmer's sudden rescue.
Promethean Knights phase in to attack the Spartans. Palmer takes aim and shoots Halsey in the shoulder.
Knights phase in. DeMarco swings at one with his sword.
The Spartans fight the Prometheans. Thorne and Palmer charge ahead chasing 'Mdama, who is carrying Halsey away. Madsen picks off two Prometheans with his sniper rifle. 'Mdama reaches the end of the room and turns to face the charging Spartans. A Knight phases in, grabs 'Mdama and Halsey, and snarls as it teleports them away.
The two Spartans stop, having just lost 'Mdama. Fireteam Majestic rejoins them.
Thorne holds up the half of the Janus Key. Fade to black.