The Fallen Star is the very first quest in Diablo III. The quest is given upon first starting the game, and has your character investigating a fallen star that has landed near the old town of Tristram, with the dead rising in its wake.
The Dead at the Gates Edit
You start out at the outskirts of New Tristram. As you approach, you will encounter Risen Dead feasting upon dead villagers. Kill them and make your way toward the gates. There, you will meet Captain Rumford, who is fighting with his men against more Risen Dead. After fighting them off, speak to him. He will tell you that the star fell on the old Tristram Cathedral, and that there was only one survivor, a girl named Leah. After that, you must help Rumford fight off an onslaught of more Risen Dead that rise from the ground to attack the gates. Once both waves of the Risen Dead have been re-killed, Rumford will direct you to the Slaughtered Calf Inn, where you will find Leah.
The Slaughtered Calf Inn Edit
Enter New Tristram and head for the Slaughtered Calf Inn. Leah will be there tending to a group of wounded survivors. Talk to her, and she will tell you that she survived the fallen star, but that it blasted her uncle Deckard Cain into the depths of the old cathedral, and that she wanted to search for him but the dead were already rising. You will then have to kill the survivors, who turn into more Risen Dead. After killing the Risen, speak to her again, and she will direct you back to Captain Rumford at the gate.
The Wretched Mothers Edit
Head back to the gate and talk to Captain Rumford. He will tell you to strike at the Wretched Mothers and their queen, who are vomiting out the Risen Dead that are attacking the town. You will have to help Rumford fight off another assault upon the barricade to the north, until one of the Wretched Mothers falls. He will then direct you to kill more of them in the ruins down the road. Head down the road and kill every Risen Dead, Quill Fiend and Wretched Mother you find until you reach the ruins of Old Tristram.
Old Tristram Edit
Upon reaching Old Tristram, continue killing Risen Dead and Quill Fiends until you find the Wretched Queen, the first elite monster you are likely to face in the game. She behaves quite similarly to the Wretched Mothers, and should go down with ease no matter what kind of character you are using. Once the Wretched Queen is dead, activate your first Waypoint located to the upper right of the map near the gates in order to return to New Tristram.
Once you make it back to town, speak with Captain Rumford. He will tell you to talk some sense into Leah, who won't give up the idea of rescuing her uncle. Your character will tell him that she shouldn't and that they will help her find him. This will end the quest. Speak to Leah to begin the next quest, The Legacy of Cain.