First, the Sparknotes summary:
Before the suitors realize what is happening, Odysseus shoots a second arrow through the throat of Antinous. The suitors are confused and believe this shooting to be an accident. Odysseus finally reveals himself, and the suitors become terrified. They have no way out, since Philoetius has locked the front door and Eumaeus has locked the doors to the women’s quarters. Eurymachus tries to calm Odysseus down, insisting that Antinous was the only bad apple among them, but Odysseus announces that he will spare none of them. Eurymachus then charges Odysseus, but he is cut down by another arrow. Amphinomus is the next to fall, at the spear of Telemachus.
Telemachus gets more shields and swords from the storeroom to arm Eumaeus and Philoetius, but he forgets to lock it on his way out. Melanthius soon reaches the storeroom and gets out fresh arms for the suitors. He isn’t so lucky on his second trip to the storeroom, however, as Eumaeus and Philoetius find him there, tie him up, and lock him in.
A full battle now rages in the palace hall. Athena appears disguised as Mentor and encourages Odysseus but doesn’t participate immediately, preferring instead to test Odysseus’s strength. Volleys of spears are exchanged, and Odysseus and his men kill several suitors while receiving only superficial wounds themselves. Finally, Athena joins the battle, which then ends swiftly. Odysseus spares only the minstrel Phemius and the herald Medon, unwilling participants in the suitors’ profligacy. The priest Leodes begs unsuccessfully for mercy.
Odysseus has Eurycleia come out. She openly rejoices to see the suitors dead, but Odysseus checks her impropriety. She rounds up the disloyal servant women, who are first made to clear the corpses from the hall and wash the blood from the furniture; they are then sent outside and executed. Odysseus tells Telemachus to cut them down with a sword, but Telemachus decides to hang them — a more disgraceful death. Last of all, the traitor Melanthius is tortured and killed. After the bloodbath, Odysseus has the house fumigated.
Not much commentary on this is needed. Time has now run out for the suitors, and Odysseus starts shooting. Eurymakhos tries to blame everything on Antinoos, who is the first to die - but to no avail. Soon, the suitors are all dead. Homer (the bard) reports that Odysseus decides to spare Phemios (the bard). Most of the female slaves are also spared, though a few of them who fraternised and collaborated with the suitors are hanged.
When the massacre is over, Odysseus makes sure to clean up after himself. This is one of the reasons that Samuel Butler thought that the poem was written by a woman.