The Kree’s, Inter-Galactic Sentry 459, protected an island in the Pacific Ocean, which was actually a Kree spaceport; termed Outpost Ten. He was a living sentry, trained and placed there many years before by his Kree masters to keep intruders out of their spaceport. But when a pair of human explorers found their way onto the island and opened the spaceport using the broken hand of a Kree idol, the Sentry awoke to guard his post. He froze the two men in a solid, mass of atoms, and shielded the island with a vibro-screen that would tear apart any approaching ship. The screen was also colored using the Kree’s color spectrum, making it invisible to human eyes.
The Fantastic Four soon traveled to the island, which Ben picked at random, using a dart and a map, and intended to vacation there. They flew using their Pogo Plane as the airlines were all booked up, and Mister Fantastic’s sensors detected the screen in time for the Invisible Girl to save them with one of her force shields. The Sentry saw them descending and ducked their plane before grabbing it by the fuselage. The four jumped from their plane and fought with the Sentry. He used an instrument panel on his chest to fire a blast ray, but the Thing swiftly smashed his panel, and he was forced to fight using only his own strength and ingenuity.
The Sentry created a tidal wave by pounding the ground with his fist, nearly destroying the Fantastic Four, until the Human Torch and Lockjaw appeared to save them. During a fight with the Sentry, Johnny missed him with a fire blast and set off the alien power source underneath the island, causing it to build to an explosion. The Fantastic Four scrambled to Lockjaw, though Ben fell through the ground and saved the two humans the Sentry had imprisoned earlier, and the group surrounded the Inhuman canine, who teleported them to safety. Left alone, the Sentry made his final report, remaining behind with the self-destructing spaceport as the price for his failure.
After some time the Sentry was somehow recreated and landed in a small town in the panhandle of Texas where he was reprogrammed by the Super-Adaptoid and joined his team of robots known as Heavy Metal. Heavy Metal eventually clashed with the Avengers and the Sentry was taken down alongside his other robotic cohorts. He was then placed in a stasis field alongside the other Heavy Metal members. A short time later they were then reactivated by Machinesmith and battled several female Avengers. They later escaped from the Avengers compound after a battle between Doctor Doom and the Avengers.
Later the Sentry was found and reprogrammed by Ultron to serve him, but was defeated by a team known as the Underground Legion who later reprogrammed it to serve them. During one of their missions however the Sentry was once again destroyed and has not been heard from since.