Kyle “Nighthawk” Richmond is injured by a car bomb intended to kill him. The Defenders assume Evil Hyperion of the Squadron Sinister did it, so they go after him. (Why wouldn’t Hyperion just vaporize Nighthawk with heat vision or hurl him into the sun?)
But it wasn’t the Squadron, of course. It was Egghead. And despite not being a member of The Defenders or having any particular allegiance or relationship with Nighthawk, Hank Pym goes after him.
We get some insight into his origin.
In the meantime, the Squadron capture the Defenders and Pym, as Yellowjacket, frees them and they all beat up the Squadron.