As viewers, we already knew the model would be returning to the TV show that made his name but his appearance on tonight's episode is a total shock to the islanders.
And in typical, laid back style, he simply strolls in and declares: "Love Island, what is up?!"
"So, I was lucky enough 10 years ago to take part in the original Love Island, and I ended up winning it, so it was a great experience, great memories," he tells the assembled group.
It seems a few of the islanders are a tad starstruck by the handsome star's appearance, with Jess gushing: "We're in the villa, going about our own business and who rocks up? Calum f**king Best!"
Calum, who most recently appeared on Celebrity Big Brother, wastes no time in bonding with the boys, announcing: "Boys, you and me are going for a night out in Magaluf! Let's go!"
As you'd expect, some of the girls are a tad anxious about what their other halves will get up to on a night out with the former party boy…
Will he lead them astray? Will they get back home in time for bed?
So many questions!
One thing we know won't happen, however, is Calum hooking up with any of the female islanders, as he's in a happy relationship with former Made In Chelsea star Ianthe Rose Cochrane-Stack.
He told one fan on Twitter last week of his appearance: "Not to pair up babe I have an amazing mrs, [it's to] share my experiences."
Love Island, ITV2, Sunday – Thursday at 9pm/10pm. Catch-up show, Saturday, 9pm/10pm, ITV2
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