It's hardly essential, but this add-on will keep fans happy.
The history behind Volition's Saints Row expansion Enter the Dominatrix is interesting, to say the least. It was originally planned as downloadable content for Saints Row the Third before THQ decided on making it a full-blown sequel instead. Then, following the scuttling of the publisher's assets – in public, mind you – it was never heard from again. That is, until earlier this year, when Volition vowed to bring it back as part of the Saints Row IV franchise. Now, here it is.
Like the rest of the Saints Row series, Enter the Dominatrix forgoes seriousness in favor of the ridiculous and adult-oriented. You're fighting in a Matrix-style version of Steelport, doing battle with the heinous Dominatrix every step of the way, and running into everything from sex slaves to news reporters. It feels more like a recap than a genuine add-on, but fans probably won't mind.
At least the add-on is self aware, with plenty of references to tiring escort missions (nailed it) to humorous energy bar woes to the conclusion, which is about as silly as you might expect. I won't dare spoil it for you, but it's a party in itself.