The Towie star is challenged to show her aggression and fighting skills by taking on the show's leader in a fight in the desert.
Luckily for Reyes, 50, he is heavily padded - because while McCann, 32, is wearing no protective gear at all, she throws herself into the fight, screaming as she pummels him.
After the fight, McCann tells the other directing staff how much she enjoyed it, while Reyes complains that his head was kicked like a football.
The reality TV star isn't the only one of the celebrity recruits challenged to a fight with Reyes, as a group of those the directing staff feel are falling behind are called in to show what they can do.
Later in the episode, McCann gets tearful as she opens up on the effects of her parents' divorce, her own difficulties in forming relationships, and her dislike of being pigeon-holed as a talentless reality TV personality.
In a directing staff interrogation she says: "I'm terrible in relationships, I don't think I've ever had a healthy relationship.
"If I start dating someone, it's very much all or nothing. It's either they're dead to me, cut them off, or I'm obsessing, do they like me?
"I really want that reassurance."
With the directing staff looking for recruits to send home this week, will McCann manage to get control of her insecurities and become confident enough to succeed?
Viewers will watch the recruits take on a terrifying challenge designed to whittle down the group and choose who to boot from the competition.
They will be driven to a port during the night and told to dive backwards into the dark water a long distance below, which they must enter head first to pass.
Celebrity SAS: Who Dares Wins airs on Sunday at 9pm on Channel 4.
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