Attack on the Helicarrier

The Attack on the Helicarrier was a battle that Loki planned to help him escape from the Helicarrier and take the Tesseract. It resulted in the death of S.H.I.E.L.D. Agent Phil Coulson.

Background
In the Destruction of Project P.E.G.A.S.U.S., Loki had returned to Earth to control it. Eventually, he was captured by S.H.I.E.L.D., using this as his way to get the Tesseract, when he escapes. With Hawkeye and Erik Selvig still under his control, he made them come to save him.

Nick Fury took Loki into S.H.I.E.L.D. custody aboard the Helicarrier, where he was placed in a confinement capsule designed to hold the Hulk. Loki taunted Fury about his attempts to control the Tesseract and his gathering of misfits to defend Earth.

Loki's mere presence was enough to provide a disruptive element that would fragment the only group of heroes that had any hope of stopping him.

Battle
When Black Widow arrived to speak with him, apparently to offer a deal in exchange for Hawkeye's freedom, he subjected her to a play of wits where he ferreted out her deeper misgivings, but was completely caught off guard when the spy tricked the trickster into revealing that there was already a monster among them and that it was not him. From this, she concluded that the monster was Banner and that he had planned to use the Hulk to destroy the ship and the heroes as well.

However Loki's plan to use the Hulk, only one aspect of his real intentions, did follow through as another jet carrying Hawkeye and the rogue agents came to his aide, following a signal from the scepter. The wounded Banner transformed into the Hulk, destroying much of the Helicarrier, while one of Loki's controlled agents freed him from the cell.

While everyone else was fighting the attackers, Iron Man and Captain America fixed the rudder that the attackers had almost destroyed.

Loki then used illusions of himself to trick Thor into entering the cell and to distract Agent Coulson. Loki mortally wounded Agent Coulson and ejected Thor's cell from the Helicarrier. Though he retrieved his scepter, Hawkeye was released from the mind control, and Loki headed to New York City, where Selvig waited with the Tesseract at Stark Tower.

Aftermath
Enraged by Phil Coulson's death, the Avengers decided to end the war Loki had started by defeating him in the Battle of New York. It turned out that Coulson was dead for days, but Nick Fury had S.H.I.E.L.D.'s scientists working their hardest to save him.