The Avengers, at the request of the UN, go to the moon to determine if the Inhumans have peaceful intentions. They go there, and at first things seem fine. However, several meteors are being launched towards earth. Thor goes off to stop them.
The Avengers have to suspect the Inhumans, as they were launched form the moon. Black Bolt declares war on the Earth and demands the execution of the Avengers. A battle ensues. However, She-Hulk manages to threaten Black Bolt, and in his panic, he speaks.
So, everybody knows that Black Bolt's body has been taken over by somebody else. It turns out to be his brother Maximus, the Mad. Somehow he managed to switch bodies with Black Bolt.
They free Black Bolt from the crystal he's encased in, and Black Bolt is able to force Maximus to switch them back. The attack on the Earth is halted, and the Avengers are able to leave in peace.