459

"459" is the 15th episode of The Avengers: Earth's Mightiest Heroes.

Synopsis
Henry Pym and Wasp go to see Jan's friend Carol Danvers, who needs Hank's help to identify an object sighted by an observatory telescopy a few days ago, and neither Carol nor chief scientist Philip Lawson believe it's natural. It turns out to be an alien robot heading to the observatory, and impervious to anything that is shot at him. Lawson uses a strange energy field to shield Carol from one of the robot's beams, which causes him to lose his human diguise and reveal he's actually the Kree Captain Mar-Vell, who orders the robot to deactivate, but the order falls in deaf ears. The robot destroys the observatory but everybody leaves alive, though Carol must be sent to the hospital.

After other Avengers are called, Mar-Vell explains the Kree and the Skrulls want to take over Earth as it's in a strategic location for the war against each other. The robot is a Kree Sentry sent to destroy any treath to the Kree occupation, and if the treath is too big, he will use his attached nega-bomb to wipe all life from the planet. Mar-Vell has grown fond of Earth and wants to help the Avengers in stopping the Sentry.

The Avengers intercept the Sentry, who has activated the nega-bomb. When the Sentry knocks Giant-Man out, Wasp gets angry and attacks the Sentry from the inside, giving Hulk the opening to open the Sentry's chest. The Sentry's deactivation triggers a Kree transmission declaring Mar-Vell an Earth sympathizer. The bomb is about to explode anyways, but Thor takes it away from Earth.

Mar-Vell says he will talk on Earth's favor to the Kree government, and leaves. In the hospital, Carol finds out the consequences of Mar-Vell shielding her earlier when she finds herself floating over her bed.

Trivia

 * Only the founding Avengers appear in this episode.
 * Carol Danvers gets her powers, but doesn't appear as Ms. Marvel until "Welcome to the Kree Empire".