I chose to start with this screenshot because before this a number of different things happened. Nik Kronwall, who you can see near the scoreboard chasing down Chris Kunitz I believe, pinched in deep to hopefully capitalize on a scoring chance. He didn't, and the shot was blocked by Brooks Orpik and sent around the boards to James Neal, who caught Henrik Zetterberg sleeping at the top of the zone, allowing the puck to get to Malkin.
As you can see, Stuart is almost about to get himself into position to defend a two-on-one. Malkin and Kunitz are both in really good position on the ice, as Stuart wasn't close enough to Malkin yet, which allowed Kunitz to catch up. Stuart is about to transition from forwards to backwards, and has already thought about where he needs to be on the ice.
Stuart is now skating backwards, and does a smart thing by quickly glancing over at Kunitz before turning his attention to Malkin. A defenseman's purpose during a two-on-one is to make sure the guy with the puck isn't able to get a pass across, thus requiring the goalie to move laterally, only increasing the chances the offense scores. He also has to keep forcing the puck to the outside to cut down the angle the shooter has for the shot. As we can see, Stuart is doing a good job of both requirements this far into the play. Malkin really has to opportunity to make a pass horizontally, and Jimmy Howard is already out of the crease to cut down on the angle.
As Malkin prepares to shoot, Stuart is still doing a great job of eliminating the pass. One of the only things I can even kind of fault him for on this play is that he may not have pushed Malkin to the outside enough before he released his shot. After he scored though...we all realized that it probably didn't matter how far outside he was, that shot was ridiculous. Regardless of the shot though, what I'm looking at is how well Stuart's coverage was on this play. It's hard to fault him at all for the goal. He's staying in between the attackers and taking away the pass.
By now it's clear to Howard that Stuart is playing to take away the pass, leaving Howard to stop the shot. This is evidenced by Stuart dropping down to prevent a potential pass. Howard knows this, and comes out of the crease before the shot. Unfortunately, Malkin unleashed a wicked wrister than beat Howard on the blocker side.
While no one was faulting Stuart for the goal immediately after the play, it's still cool to look at it as the play developed and see how well his coverage was. Ultimately I'm not sure he could have played it any better. Malkin's shot was just that good.