Alain Prost was on pole with Nigel Mansell joining him alongside. After a 45-minute rain delay, (the track was so wet that the drivers requested that the officials water down the tunnel section because the soaking wet cars would be more difficult to drive on a dry surface with wet tyres) Senna started a lowly 13th place. On lap 9, Mansell passed Prost to take the lead, but crashed out six laps later on the run up to Casino Square after slipping on a painted white line and damaging the car on the armco. By then Senna was third, behind the two McLarens. On lap 19, Senna passed Niki Lauda for second, and just to add insult to injury, would suffer the ignominy of going off five laps later; proof that even F1 legends were struggling with the atrocious conditions. At this point, Prost was still 30 seconds ahead, but Senna was rapidly catching him, despite his inferior Toleman. On lap 29 and again on lap 31 Prost waved to the stewards of the race to indicate that he felt the race should be stopped. The red flag to stop the race was shown at the end of the 32nd lap after clerk of the course Jacky Ickx decided that conditions were too poor for the race to continue. Senna passed Prost's slowing McLaren before the finish line, but according to the rules, the positions counted are those from the last lap completed by every driver - in this case lap 31, at which point Prost was still in the lead.
As an aside, the stoppage was controversial; many felt it benefited Prost, driving a car with a Porsche-designed engine, as Ickx was under contract with Porsche as a group C sportscar driver. Ickx was suspended from his race control duties as he had not consulted with the stewards before making the decision. Because the race had not been allowed to continue beyond 75% race distance, half points were been awarded; had it gone beyond 75%, Prost could have had 6 points from a 2nd place instead of 4.5 points from the win. Prost would eventually go on to lose the championship to Niki Lauda by half a point.
But this is not to take away from the sheer brilliance of Senna; to take the TG184 and potentially win the race with it, which to translate to 2012 would be like taking a HRT or Marussia to the giddy heights of a podium finish. As James Hunt prophetically stated in his race commentary about Senna: "He's got a big future."
How right you were, Mr Hunt.