Hels had finally caught Impulse and kept him long enough to get married. In Hels law, the more powerful being in the relationship controls the whole thing. Helsy used that to plan a big old ceremony to show off Impy to everyone as a way of not only claiming him in front of basically everyone in the whole dimension, but also showing the whole dimension who is in control in the relationship. He even got Impulse to wear a fancy dress and a veil that basically obscured his face.
The problem arose when he lifted the vale for the ceremonial kiss only to find Zed's face under it. After everyone in the dimension stopped screaming and begging for their lives, Zed reviled Hels had had him instead of Impy the entire time.
Hels then remembered the rule about the more powerful being having more control in the relationship.
(Zed only really uses this to get Hels to leave Impy alone.)
Truthfully, Zed wishes it hadn't come to this. He's away from his friends, he's stuck with Hels and stuck in this stupid power dynamic. He'd try and make fun from the situation - pretend to be a spy undercover (technically he is) - but he's just miserable. He's miserable having to wait for the wedding, he's miserable watching everybody scream because of him, he's miserable afterwards too.
But, Impulse had been so scared. He tried not to show it, because he's Impulse, but Zedaph could tell. Even Tango could, and you know something has to be bad for Mr. Oblivious to notice. Impulse was barely sleeping, always glancing behind him, making sure curtains were closed. He just... Looked sad.
So, Tango and Zed came up with this plan. They disguise Zed as Impulse (not too hard, their builds are fairly similar) and let Zed get captured in his place. Tango knows the rules of hels ensures that Zed will have power in the relationship and he can use that to get Hels to leave Impulse alone!
The plan worked. Somehow. But now Zed wishes they thought about what comes next.