Erm, well the mystery lady (Jane Dobinson iirc) claimed to be working for the British Secret Services and gave Mark a series of tasks to perform, promising sexual favours and huge sums of money in return. She convinced Mark that his friend, John, was in fact one of the most important people in Britain as only he (along with the queen) had access to a safe kept at the bottom of the atlantic ocean (!). I can't remember what was supposed to be in the safe.
This continued until Jane Dobinson told Mark that his final assignment would be to stab John. She promised that once this was done, she would be the first to arrive on the scene (disguised as a policewoman) so that he would not be arrested for the crime, and following this she would sleep with him (and pay him millions of pounds).
Mark carried out the stabbing, almost killing John. The police soon worked out that Mark had to be the one who stabbed John (from CCTV footage), but couldn't work out the motive. Mark held out for a month waiting for Jane Dobinson to vindicate him, but eventually told the whole story to the police.
The police examined Mark's computer and found the chatroom scripts were stored on the hard drive. On examining the conversations that had taken place between Mark and Jane Dobinson, Rachel (the murdered girlfriend) and Patrick (the gay stalker), they discovered that there was a spelling mistake common to all three - "maybe" was misspelt "mybye". This enabled them to identify John as the person behind all of these personas. John had created the whole thing, Mark had been extraordinarily gullible, and (according to the barristers) what followed was known in legal terms as a "folie a deux" - the combination of the two boys' characters resulted in a unique madness.
Mark was charged with attempted murder, and John was (uniquely) charged with incitement of his own murder. Both avoided custodial sentences, but are not allowed to use the internet unsupervised, and are never allowed to meet again.
Surely it's only a matter of time before the movie comes out!