Back in Gryffindor Tower, Harry realizes the urgency of stopping Voldemort and explains the situation to Ron and Hermione. He then goes to bed, unexpectedly finding his Invisibility Cloak back in his bed.

Detention in the Forbidden Forest
Filch takes Harry, Hermione, Neville, and Malfoy down to Hagrid's hut to serve their detentions in the Forbidden Forest, lecturing them as they go.
Hagrid leads Harry, Hermione, Neville, and Malfoy to search for the killer of a unicorn in the Forbidden Forest. He splits them into groups and sends Neville and Malfoy off with Fang.
Walking through the Forbidden Forest in search of a unicorn, Harry, Hermione, and Hagrid encounter the centaurs Ronan and Bane, who are rather unhelpful.
As Hagrid, Harry and Hermione leave the centaurs, Hagrid spots [wand sparks] from Draco and Neville and runs off after them, leaving Harry and Hermione concerned behind him.
Hagrid returns upset with Malfoy and Neville, and sends Harry off with Malfoy and Fang instead.
Harry and Malfoy find the dead unicorn, and see something drinking its blood that makes Harry's [scar] burn. Malfoy runs and a centaur shows up to chase off the mysterious cloaked figure.
Firenze invites Harry to ride him to safety, infuriating Bane. He takes him to Hagrid, however, explaining unicorns and [unicorn blood] - and Voldemort's desire for the [Philospher's Stone] - to Harry along the way.
Chapter 16 - "Through the Trapdoor" - In which the students begin their exams, Hagrid lets slip how to control Fluffy, HRH try to warn Dumbledore but he has been summoned away. Fearful that Snape is about to make his move to get the Philosopher's Stone, HRH decide to go after it themselves. Under Harry's invisibility cloak they make their way to Fluffy, put him to sleep, and go through the trapdoor, past plants, chessmen, and potions.
Exams and Other Worries
The first years get through their exams, and though Harry still thinks constantly about Voldemort, Ron and Hermione grow less worried over time.