The ferris wheel operator and the freed prisoner were in on it together, since they both met at an AA meeting. The couple tries to escape, but Monk finds out, and when he gets to the park, the operator is stabbed. Sharona and Adrian wander around the park to find the prisoner, and Sharona gets on the ferris wheel to find him, only to find that he's on the ferris wheel too, coming towards her. The Captain eventually gets there, not before Monk tries (and fails) to save Sharona from the prisoner.

The motive for the prisoner was that the cop accused to stabbing the guy was also the only witness against him in his trial - ergo, if the officer was behind bars, he'd be set free. The prisoner hired the dead guy to help him to do so, but the guy only expected to get beaten up, but the operator was also in on this, therefore completing the circle.