Mendes via KO in round 4. Events transpire as stated below.
Fight
Rd 1: Mendes hangs with Conor in the standup and laughs off some taunting. Conor slips in a few flashy kicks to the body, which Mendes shrugs off. Things are relatively uneventful until about halfway through the round where Chad takes Conor down on a failed roundhouse attempt. Chad controls Conor and rains punches for a 10-9.
Rd 2: Mendes, now more comfortable with Conor's style, takes Conor down repeatedly and this time beats Conor badly for a 10-9, possible 10-8.
Rd 3: Mendes continues to beat on an exhausted Conor McGregor. Conor survives the round on his elbows and knees in a turtle position, while a ref reluctantly allows Conor to continue to the end of what is likely a 10-8 round.
Rd 4: Chad continues to beat an exhausted McGregor who simply has no answer for Chad's onslaught. Chad does some obnoxious showboating (winding up punches) against his helpless foe, drawing boos from the crowd. Eventually Chad lands an underarm punch, knocking McGregor unconscious. The camera cuts away at a shot of Conor's legs convulsing. Though the crowd is loud and enthusiastic, the end is rather merciful and anti-climactic. It is clear to all observers that Conor should not have been in there with Chad.
Post-Fight
In his post-fight interview, Chad will be confronted with his disrespectful showboating. "Yeah, I'm sorry about that. I guess I just kind of let the fight go to my head. That was not cool of me. All respect to Conor though, he's a great competitor. I'm sure we'll be seeing him again real soon. Right now I'm just trying to stay focused on that belt. So, Jose, if you can hear me... heal up brother. Money Mendes is back, and he's looking for you."
Conor, who is under observation at the hospital and not present at the post-fight conference, releases a statement on Twitter later that night. "You can't win them all. These things happen. Respect to Chad. I will be back stronger."
Epilogue
3 days later, Conor tests positive for banned substances. The UFC denies any knowledge of this, although confidential sources contradict this claim. A investigation is launched. And that's as far as my crystal ball can see.
You heard it here first.