On the bitter cold ramparts, Hamlet, Horatio, and Marcellus keep watch. Meanwhile, from inside the castle they hear the roar of revelry. Hamlet condemns Claudius's constant merry-making, saying that it makes the noble Danes look "swinish" and corrupt.
Hamlet connects indulgence of desires to corruption. What looks like enjoyment only hides internal corruption.
The Ghost appears and beckons Hamlet to follow it. But Horatio and Marcellus hold him back: they think the ghost may be a demon laying a trap for him.
Religion provides no answers: what looks good could be evil.
Hamlet breaks free of them and follows after the Ghost.
Hamlet takes decisive action.
Marcellus says "something is rotten in the state of Denmark" (1.4.90). They run after Hamlet.
The nation suffers for the immorality of its leaders.