Sleeper Mechs infiltrated the Principia, which was floating in the air, to secure a Cybertek shipment of Gravitonium. After the majority of the sample was safely transported off of the ship, several robots were left dormant in the ship as sentries, waiting to ambush any potential future intruders.
During the Infiltration into St. Louis Aerospace Facility, the mechs accompanied General Hale's daughter Ruby to ambush the S.H.I.E.L.D. fugitives, who were distracted by Piper. They held agents Yo-Yo Rodriguez, Melinda May, and Alphonso Mackenzie at gunpoint. On orders from Ruby, they attempted to execute the agents but were relieved of their weapons by Rodriguez. In reaction, the robots charged to engage the agents in hand-to-hand combat, holding S.H.I.E.L.D. back before the agents managed to escape, albeit with the loss of Rodriguez's arms.
Several Sleepers accosted Werner von Strucker when he threatened Ruby. Once Strucker reached an understanding with Hale and ceased hostility, the mechs forcefully escorted Strucker to his room.
The Sleepers left on the Principia were activated after sensing S.H.I.E.L.D. agents Alphonso Mackenzie, Daisy Johnson, and Phil Coulson on the ship. They awakened and attacked the agents, but were easily defeated before the ship plunged out of the sky. However, one mech was taken from the ship to the Lighthouse by Mackenzie.
- In the comics, the Sleeper was a robot built by the Nazis on Adolf Hitler's orders. It was commandeered by the Red Skull after World War II who wanted to use it in his conquest of the world.