It looks like Nikolai is there on the worst day possible to intercede with the sovereign, as it's the day of the peace, the 27th of June. There's a banquet planned, and flags crossed in the streets with A and N (for Alexander and Napoleon).
Nikolai can't stay with Boris, and doesn't want his help (it's clear from the last chapter that he's going to be no help), so Rostov decides to take the letter to the soverign himself, in a fit of bravery (stupid or not).
He walks right into the house the emperor occupies, and is stopped by an officer who tells him to go downstairs to the officer on duty, but he won't be received.
Sure enough, when he gets there, he catches the valet dressing the officer in his finest suspenders and tunic. He's talking to someone else, who says to come back, and is even more shocked and disturbed that Nikolai would not go through the chain of command.
Luckily (as this is a book), he runs right into the cavalry general he knew from when he first saw the soveriegn. The man agrees to talk to the Tsar about it, since he knows Denisov is a good man.
Sure enough, the sovereign comes out of the house, and Nikolai is again dumbstruck, as the tsar was "illuminating everything about him with his gaze." The general speaks with him, and then as he mounts his horse, he says loudly, "I cannot, General, and the reason why I cannot is that the law is stronger than I".
Things are not looking up for Denisov. It looks like, no matter how noble he was in trying to feed his men, he'll be disciplined for it, if he's not dead yet. Although Nikolai is overwhelmed by the Tsar, who is so far given nothing but god-like qualities, it's clear that Tolstoy is beginning to expose the ridiculousness of those in power, and in some sense the powerlessness they have. The tsar has real power, but also at the mercy of what everyone around him thinks of him. He must be a political creature. And, in being so, Denisov may die and Nikolai will be disappointed. It seems from the glowing terms in which the tsar is described here, though, it will be hard for Nikolai to think any less of him. That's probably why he said what he said as loudly as he did. There's no way Nikolai will fall out of love, no matter who it's for.
8 months ago