Now we're at the party, and Hermione is hiding from her date—who turns out to be that dreadful McLaggen kid. She admits to Harry that she thought that would bother Ron the most. Well played.
Snape comes to give Harry a message from Dumbledore: the headmaster wants Harry to have a good holiday. Dumbledore is going to be traveling for a while, but Snape won't tell Harry where.
Suddenly, Filch interrupts the party by dragging in Malfoy. He tells Slughorn he found Draco hiding in the hallway.
Malfoy "admits" to trying to crash the party (liar!), and Snape offers to escort Malfoy back where he belongs.
Then, out in the hallway, we hear/see Snape talking to Malfoy. Snape asks Draco to let him help with whatever this mission is, but Malfoy claims he's a big boy been chosen for the job, and he doesn't want Snape's help—it's his moment, and he won't fail.
Snape mentions the Unbreakable Vow he's made to help Draco…and since Harry's eavesdropping on their sinister little convo, now he knows about the promise.