Each month(ish) I'm revisiting one of the big comic book crossovers of the last few decades. For my broad rules on what counts for consideration, see here.
The late noughties was a turbulent time in the Marvel Universe. The Avengers had been disassembled then brought back New, the mutant population had been decimated in House of M, and the Civil War had split the heroes down the middle into pro-registration/authority and outlawed anti-registration factions (I’ll cover Civil War down the line). Most particularly, the flagship franchise, the Avengers, were seriously at odds. Tony Stark, now Director of SHIELD, was leading the officially sanctioned team, and Luke Cage’s underground New Avengers were still reeling from the death of Captain America in the aftermath of Civil War.
In the middle of this ongoing conflict, the New Avengers, engaged in a mission in Japan, encountered the assassin Elektra in a fight which apparently cost her life. Except that it wasn’t her. On her death she reverted to her natural form as one of the shapeshifting race of Skrulls, but a Skrull impervious to detection by any means; physical, mystical or technological. This immediately set the heroes on edge, aware that such infiltrators could be anywhere. Jessica Drew (Spider-Woman) arguing that they had a responsibility to tell Stark and the authorities about the threat, took the opportunity to betray her team and take the Skrull to Stark - joining his team in the process. All this set the scene for Secret Invasion.