Harry wasn’t stupid, he knew the tournament was another ploy, yet this time he knew it wasn’t Dumbledore pulling strings because he saw, watched, listened as Voldemort planned it. A port-key. Simple, and rather ingenius, it was the focal point, so much so that no one assumed it could be tampered with. So when he saw Cedric close to the cup and heard Cedric's preposition, he agreed, but as they landed, and before Cedric could process anything Harry shot a sleeping spell at him, stunned him, disillusioned him, then laid his cloak over him and the cup.