This is not just a plugin, it symbolizes the hopes and ethusiasm of an entire meme, summed up into one word, made famous by the internet: RICKROLL.
The Rickroll plugin converts your embedded videos into Rick Astley's "Never Gonna Give You Up." Activate it and that's what it does. Deactivate and they go back to normal. If there's no Rickroll video on the embed's site, it defaults to YouTube's 'official' RickRoll video.