The long-anticipated lie detector test will put a strain on relationships in the Love Island villa.
Dani Dyer will try to establish if Jack Fincham is a trustworthy boyfriend when she grills him as he is hooked up to a machine.
The couple have been together since the beginning of the series and are favourites to win the £50,000 prize at the end of the ITV2 dating show.
The daughter of EastEnders star Danny Dyer will attempt to find out if the stationery salesman is interested in her or just her famous father when the girls are given the power to draw up questions for the boys.
In scenes that will air on Monday’s episode, she tells the Beach Hut: “I’ve been cheated on, I’ve been lied to. I’ve had it all.
“It doesn’t bring a nice side out in me. Loyalty, it means so much to me.”
She asks him if he has been attracted to any other girls on the show, if he wishes she was sexier, if he feels he settled too quickly and if he coupled up with her because he knew who her father was.
She also asks if he is in love with her and if he could be tempted by other girls outside of the villa.
His answers cause tension between the couple and when Dani expresses her concerns, Jack tells her: “It’s just annoying me that you’re now talking about all the negative ones.”
Dani replies: “Any girl in the world, if they’ve got that question and their boyfriend got it wrong, they would be upset.”
The test also reveals the insecurities former stripper Megan Barton Hanson has about her relationship with Wes Nelson.
Speaking before he does the test, she says: “It could go one of two ways. It could really knock my confidence more and I’m trying to be positive and trying to trust someone finally after I’ve been hurt in the past.
“Or it’s just going to prove me right and be like, ‘See, all men are the same.”
Megan adds: “It’s a once-in-a-lifetime opportunity to get him hooked up to a lie detector, I’m not messing around.”
She asks if he thinks he will struggle to stay faithful to her with all the attention he will get outside, if he thinks she is wife material and if he is embarrassed to take her home to his parents.
Megan admits she is concerned that the design engineer’s parents might not approve of her past life.
She says: “It’s like, ‘Are you embarrassed of my past?’ and, ‘Do you want me to meet your parents?’ all in one.
“If I was someone’s mum, I would want them to bring back sensible Sally that’s never got her tits out for a living.”
The test will also see Laura Anderson grill Paul Knops about whether she is a better kisser than Britney Spears, after learning the carpenter once appeared in a music video with the pop star.
And the burgeoning romance between Alex George and Alexandra Cane, which appeared to blossom after a night in the hideaway, could hit the rocks under the strain of the lie detector.
Alexandra breaks down in tears, saying: “I think you just always want the person you like to like you back in the same way. I just know that I really like Alex.”