Madam Pomfrey keeps Harry over the weekend. He spends his time mourning his broken broomstick and seeing visitors, though nobody is able to make him feel better. Harry doesn't tell anyone about the Grim and keeps quiet about what happens to him when the dementors get close—Harry knows now that he hears Lily screaming and Voldemort laughing as he murders her.
The fact that Harry hears his mother dying suggests that time and his memories of what happened aren't as distant as Harry might have thought. This shows that the dementors' true power is condensing time and filtering a person's conception of time to leave them with only the bad moments.
Harry is relieved to get back to normal life on Monday, though Malfoy taunts Harry by imitating Harry falling off during the match and, during Potions, he mimes dementors. Ron throws a crocodile heart at him and Snape takes 50 points from Gryffindor. Fortunately, Lupin is back to teach Defense Against the Dark Arts, though he looks ill still. As soon as the class is seated they start complaining about Snape and their werewolf homework. Lupin smiles and excuses them from the essay; only Hermione is disappointed.
For Hermione, not doing assigned homework—even if it is from Snape and not fair—is unthinkable. This is because she still believes in the sanctity of adult authority figures and while she's willing to question Snape on a personal level, she's unwilling to go so far as to challenge him as a teacher in charge of her grades.
Lupin asks Harry to stay after class. He offers condolences for Harry's broomstick and sighs that the Whomping Willow was planted the year he arrived at Hogwarts and is a vicious tree. Harry asks if Lupin heard about the dementors and then asks why the dementors affect him so much. Lupin tells Harry that he's not weak; he just has a darker past than most. He explains that dementors are foul creatures that drain positivity out of the world. If given the chance, a dementor will feed on a person's happiness and leave them with only the worst experiences of their life. Quietly, Harry admits that the dementors make him hear Voldemort murdering Lily.
Everything about this conversation suggests that Lupin is beginning to take on a mentorship role for Harry, while Harry is learning to trust Lupin even with extremely personal information. Again, Lupin's willingness to give Harry unabridged information about the dementors shows that he believes Harry is both mature enough to hear the truth and deserving of hearing it. This shows that he recognizes that Harry is growing up.
Lupin awkwardly almost touches Harry's shoulder and then curtly says that the dementors are getting angry since Dumbledore won't let them on the grounds. He says that at Azkaban, which is on an island, the dementors are so successful that the prison barely needs walls or water to keep prisoners in—most of them go mad. Harry and Lupin briefly discuss Sirius Black’s escape, and then Harry remembers that Lupin was able to deter the dementor on the train. He asks if Lupin will teach him to fight dementors. Lupin agrees, but says they need to wait until the new year.
The way that Lupin describes the prison suggests that happiness and camaraderie are essential if a person wants to remain truly alive, since dementors deprive a person of both of those things. When Harry asks for help in learning to deter the dementors, it again shows that he trusts Lupin and knows that Lupin will tell him the truth and treat this request fairly.
This promise, combined with Hufflepuff losing to Ravenclaw in a Quidditch match, makes Harry feel much better. Two weeks before term ends, it begins to snow and feel like Christmas. Both Ron and Hermione decide to stay at Hogwarts over the Christmas holidays, and everyone but Harry is thrilled when the school arranges another Hogsmeade trip for the last weekend of term. Harry decides to spend the day researching broomsticks to replace his Nimbus.
Ron and Hermione's decision to stay at school is intended to make Harry feel less alone, especially since he has no other choice of where to go for the holidays. This shows that gestures of friendship don't have to be grand; they can be as small as simply being there for someone.
On the morning of the Hogsmeade trip, Harry walks Ron and Hermione to the doors but before he can return to Gryffindor Tower, Fred and George wave him into an empty classroom. Fred pulls out an old piece of parchment and George explains that when they were first-years, they got in trouble with Filch. While he was threatening them, they noticed a cabinet labeled "Confiscated and Highly Dangerous." George set off a Dungbomb and Fred grabbed the parchment out of the cabinet. George taps the parchment and says, "I solemnly swear that I am up to no good." A map of Hogwarts appears. The top reads that Moony, Wormtail, Padfoot, and Prongs present the Marauder's Map.
It's important to keep in mind that Fred and George take a very dim view of "justice" as espoused by those in charge of Hogwarts, like Filch. As far as they're concerned, what Filch says is less meaningful or important than whatever disallowed thing they want to do. This shows Harry that it is possible to put himself above the law and work around it, while this map is implied to be the ticket to doing so.
Harry looks at it and sees that throughout the castle, there are dots labeled with names. It also shows a number of secret passages into Hogsmeade. Fred points out several that Filch knows about, one that starts under the Whomping Willow, and one that leads into Honeydukes. The entrance is right outside the classroom they're in. They tell Harry how to clear the map and say that they'll see him at Honeydukes. Harry feels excited until he remembers that once, Mr. Weasley warned him to not trust something that can think if he can't see where its brain is. He reasons that he's not using it for anything too bad, so he consults the map, which tells him how to open the hump of a stone witch outside the classroom, and climbs in.
Mr. Weasley uttered that phrase in reference to Tom Riddle/Voldemort's diary, which was an evil object and, importantly, was something that only Ginny interacted with. In the case of the Marauder's Map, it's telling that both Fred and George use it, and that they choose to pass it on to Harry. Simply because the map has been handled by so many people, it seems less nefarious than the diary did—and it's also telling that it was created by four people, not one.
Harry slides down a long slide and then follows a dark passageway. He begins to climb stairs until his head hits a door. He slowly climbs into a cellar and as an employee comes down to fetch a box of Jelly Slugs, Harry sneaks up the stairs and into the shop. He finds Ron and Hermione, surprises them, and then tells them about the map. Ron is enraged that Fred and George never gave it to him, while Hermione insists that Harry hand the map in. She thinks that Sirius Black might be using the passages and Harry wonders if Black knows about the passage to Honeydukes. Ron, however, points to a notice saying that dementors patrol the streets of Hogsmeade at night and says that Black doesn't stand a chance.
It's worth noting that Ron is right—Black is, at this point, hiding in the Forbidden Forest, not in Hogsmeade. It's perfectly logical to expect that Black wouldn't be able to sneak into the Honeydukes tunnel without raising suspicion. This shows that at least in times like this, when Ron's happiness is at stake, he's able to think logically and critically and come to reasonable conclusions given the information in front of him.
Harry, Ron, and Hermione walk through the village and decide to get butterbeer in the Three Broomsticks. Blushing at the sight of the bartender, Madam Rosmerta, Ron goes to fetch drinks. The trio sits at a table in the back next to a Christmas tree. Just as Ron returns with the butterbeer, they see McGonagall, Flitwick, Hagrid, and Fudge walk through the door. Hermione and Ron shove Harry under the table, while Hermione moves the Christmas tree in front of their table. Harry watches the adults sit at the table next to theirs through the branches.
By keeping Harry from being seen by the teachers, Ron and Hermione ensure that he (and they as well) won't get into trouble for sneaking out. This ensures that Harry will be able to use the Marauder's Map again and that he'll have his friends to thank for that. However, it's also worth noting that Harry's trip to Hogsmeade does ignore its potential dangers; he's still young and naïve.
Harry watches Rosmerta's feet as she brings drinks to the table and Fudge invites her to join them. Fudge is in town because of Sirius Black and assures Rosmerta that the dementors are a necessary precaution. McGonagall and Flitwick both express anger that they want to get inside the school, but Fudge delicately sidesteps. Thoughtfully, Rosmerta says that she knew Sirius as a boy and he never seemed the sort to go to the Dark Side. Fudge and McGonagall ask if she remembers Sirius's best friend, James Potter. At this, Harry drops his tankard.
When Fudge sidesteps McGonagall and Flitwick's concerns, it shows that he believes the dementors are good and necessary, even if they are dangerous. McGonagall's concerns, however, suggest that she prioritizes her students' safety over anything else. Learning that James was friends with Sirius Black tells Harry that adults are keeping things from him.
McGonagall, Flitwick, and Fudge tell Rosmerta that James trusted Sirius—Sirius was his best man and is Harry's godfather. Fudge says that when the Potters went into hiding, Dumbledore suggested they use the Fidelius Charm, which hides a secret inside a single person. Black became the Potters' Secret Keeper, though Dumbledore was concerned that someone close to the Potters was working with Voldemort. Within a week Black betrayed his friends, which backfired when Harry destroyed Voldemort.
The implication that it's been purposefully kept secret that Black is Harry's godfather suggests that the adults knew that Harry would become far more emotional about the whole thing knowing that Black betrayed his parents. What the adults believe also shows that they can easily follow Ministry policy without having all the information.
At this, Hagrid begins to shout and says that he was the last person to see Sirius, at the Potters' house when he went to fetch Harry. Sirius had asked for Harry and when Hagrid insisted on following Dumbledore's orders, Sirius lent Hagrid his flying motorbike. Fudge continues the story and says that Peter Pettigrew found Sirius before the Ministry did. Rosmerta remembers Pettigrew as an untalented follower of James and Sirius. Pettigrew found Sirius on the street and Sirius blew him and the street up, and then laughed.
Given what Harry learns in the Shrieking Shack, it becomes clear that Pettigrew used people's opinion of him as an untalented follower to his advantage—it made it look even more like Black murdered him in cold blood, and that he was trying to do the right thing by standing up to Black. This shows that he expertly manipulated the high emotions of the time.
Rosmerta asks if Sirius Black is actually mad, and Fudge says he's not sure. The last time he was at Azkaban, Black seemed normal and asked for Fudge's paper. Fudge says that they just hope to catch him before he finds Voldemort. With this, the teachers decide to head back to Hogwarts.
By questioning Black's mental state, Fudge is forced to accept that the dementors might not be as successful as he'd like to think they are, since he and others imply that the whole point is for prisoners to go mad.