The mum-of-three – clad in a skimpy leopard print swimsuit - dived into a swamp filled with cockroaches and fish guts to win treats for fellow contestants.
She did so by swimming through the filthy waters to retrieve seven balls.
"I knew there was no way they would just let me walk back into the camp. I knew there was going to be some kind of challenge," said the brunette, who revealed she is back on the show to find closure in her split from Peter Andre.
Despite already voicing their reservations about co-habiting with Katie, her camp-mates gave her a warm welcome with kisses on the cheek.
The 34-year-old did her best to get on with everybody, and even apologised to designer Justin Ryan about an earlier meeting.
"I'm so chilled. I'll just go with the flow," she told the other celebrities. "I don't care if I don't win. The show's not about me, it's about all of us."
Meanwhile, writing in his weekly magazine column, Katie's ex-husband Peter explained his reasons for not going back on the show where the couple met in 2004.
"I was approached by ITV to do the show three months ago, but I said no straight away," he said.
"For me it would be like going backwards and – as much as I like the people at ITV – I am keen to move forwards.
"And, more importantly, I can't contemplate leaving the kids for a month."