There's not much doubt that Michael Phelps is going to leave London having made history.
He's three medals away from the all-time record of 18 and he's swimming seven events, so injury is the only thing that will keep him from grabbing that one and likely extending his record for the most gold medals of all time. Phelps will also be trying to win a third straight gold medal in four events, something that no swimmer has done even once and which would put an exclamation point on a staggering run of dominance that we aren't likely to see again.
If he pulls those things off, Phelps will have a very strong argument as the greatest Olympic athlete of all time. Plenty of people have dominated once, a select few have dominated twice but no one has dominated three straight Olympics in the way that Phelps threatens to do in London.
Should Phelps pull it off, he will be writing the final chapter of his Olympic career with a performance that will strengthen that argument a great deal. Assuming they both make the finals (and it's about the safest assumption you'll ever make), Phelps will have to beat Ryan Lochte twice over the course of the meet.
They are both swimming in the 200 and 400 meter indvidual medley, races that Phelps had ruled with an iron fist until Lochte started beating Phelps in the last few years, including an Olympic Trials win by a margin of almost a second. Phelps only beat Lochte by .09 seconds in the 200 at that meet, an eyelash that could go the other way with a slightly slower start or turn from Phelps.
Lochte won four medals in Beijing so it's not like he's come out of nowhere to challenge Phelps. He keeps getting stronger, though, and could come out of these Games as a legend if he beats Phelps twice.
If Phelps were to wind up with silvers in both of those races -- Phelps and Lochte are miles ahead of the rest of the field -- it wouldn't do much to tarnish that already magnificent legacy. He'd still be the choice of many for the intangible honor of Greatest Olympian and it would be an easily defended choice by anyone who chose to make it.
Beating Lochte, though, would make it almost impossible to argue for anybody else. Phelps would have beaten his most serious challenger at the end of his career on the biggest stage, an irresistible closing statement in the kind of mythical contest where narratives loom large.
The 400 final is on Saturday, so we'll get an idea of which way that narrative is going to go without much waiting. By the end of the 200 on Thursday, the entire story will have been written.