The Avengers get a message that a giant green experiment just escaped a military lab. But when they go, Tony notices that the green guy is wearing some kind of harness which keeps zapping the poor guy. He talks Hulk down and gets the thing off, and I think the Hulk turns back into Bruce.
Tony decides he will not give Bruce back, screw you very much Ross.
I'm fairly sure it's on the kink meme somewhere, but maybe it's on ff.net or ao3, I have no idea at this point.