Exit the tram and head left. Follow path until you get to two doors. Only one is unlocked. Enter the unlocked door to the left.
There should be two elevators. One of these is unlocked. Enter the elevator on the right and use it.
Exit the elevator. Enter the big room. Talk to Captain Ventralis. When Shepard leaves the conversation, 2 Rachni soldiers attack.
After the attack follow the hallway. Enter the big room and head north. There should be two doors at the north end. Take the first door to the left.
Follow the hallway in to Medbay. Talk to Dr. Zev Cohen.
Head back to Captain Ventralis and talk to him.
Head back to the big room. Take the first door on the left. Follow the hallway. Enter the elevator and ride it to next level.
Exit the elevator and follow the hallway. When Shepard enters the room head left and go through door. Go to the back of room and use the Make Cure.
After Shepard makes the cure, Alestia Iallis will start up a conversation and then attack.
Head back to the elevator. On the way Han Olar will tell Shepard the Asari came from the locked door to maintenance.
After talking to Olar, head back to the elevator. Ride it back up. Head back to Dr. Cohen. Talk to him.
After the conversation, go back to Quaratine Labs. Enter the door across the room.
Follow the cave. When Shepard reaches the Ventilation Room, a Rachni soldier attacks. Enter the door to ventilation room.
Follow the hallway. When Shepard gets to the two doors, take the right one. Save the game first. Matriarch Benezia is in there. When Shepard gets near her she'll talk. After the conversation she'll attack.
This fight is fairly simple in plan. Attack all enemies she spawns. When they die she'll summon more. When she does, she expends energy. When she runs out she'll talk again.
She'll attack again after the conversation. This time Shepard can shoot her. Do so. She drops pretty fast. She'll talk again before she dies.
After she’s dead go near the Rachni Queen. A cut scene will begin. Then the Rachni Queen will talk through one of the Asari.
After the conversation there are three lock crates in the area. Head back to tram station and take door to the right as Shepard enters it. Now the left elevator is unlocked. Use it.
Exit elevator and follow the hallway. Talk to Yaroslev Tartakovsky. He tells Shepard to arm the Neutron Purge. After doing so a Rachni Soldier kills him.
Search Tartakovsky's body for purge codes.
Enter the door at back of room. Use the Mira Terminal. Tell her to initiate the Neutron Purge.
After the conversation Shepard has 120 seconds to reach the elevator. Several Rachni soldiers appear in big room. Kill them. After battle run for elevator and use it. At the top head back to the tram. Use it. Shepard will be back on Normandy. Talk to the crew and the council. Go around talking to crew members. Then use the Galaxy Map to start next Mission.