When Shepard repeatedly talks to Garrus while aboard the Normandy, asking about his history in C-Sec, Garrus eventually tells a story about Dr. Saleon. Shepard will then get the assignment to track him down, along with the coordinates of Dr. Saleon's ship, the MSV Fedele located in the Herschel system of the Kepler Verge.
This is a fairly short assignment with only one group of organic enemies who like to engage in close range. Sledgehammer Rounds and anything else that can knock them back and keep them from swarming you will be useful.
Squad-wise, Garrus, of course, asks specifically to come along, but it is up to you whether to bring him.
Walkthrough EditTest Subjects, which are more or less Thorian Creepers. However, even if you have completed Feros, they aren't vulnerable to the Anti-Thorian Gas grenades, so you will have to take them out the old fashioned way, with gunfire and physics-based attacks.
There are also a few containment canisters in the main hold whose explosive bangs can help to keep Subjects at a distance – tech abilities like Overload and Sabotage, which don't do much when aimed directly at the subjects, can be brought into play here to create larger explosions.
To stay alive, since the Subjects have the same attacks as the Creepers, keep on the move and don't let them get too close. If they do get too close they will either use their spitting attack, which results in toxic damage, or they will melee attack you. Both modes of attack cause a lot of damage, so again keep your distance and use powers and shotguns to keep them off their feet. Once all the Test Subjects are down, head towards the cockpit section of the freighter, there are no crates in the main hold. When you reach the cockpit section, the door on the left is locked, the cockpit contains nothing, and the one on the right is where Saleon is. Open the door to continue.
When talking to Dr. Saleon (or Dr. Heart, as he now calls himself), he will first thank you for rescuing him, but then Garrus will confirm his identity. Saleon will deny it, and you can ask for confirmation. Garrus makes some comments, and Saleon asks Shepard to keep Garrus away from him. You can now choose to either tell Garrus to back down, or to finish the Doctor.
- Telling Garrus to back down will lead to him objecting, but you can remind him that no one knows what he’s been up to. The squad will take him back, turn him over to the military, and get some answers. Garrus first objects, then sees why. He tells Saleon that he is one very lucky salarian. Saleon thanks them in a sarcastic tone, then pulls his gun.
- Telling Garrus at any point to shoot him, or you say that you will take him yourself, leads to a firefight.
Saleon will turn hostile either way. Killing him requires just one shot, usually before he can even shoot back. The ending conversation depends on your choices above.
After Saleon is dead, you get the following text about his experiments:
“Saleon’s medical equipment is stained with the blood of many species. Pale blue, violet, orange… and more than a few dark red. But his work has ended here. Time to head back to the Normandy.”
Before leaving, there is a Hardened Storage Locker (hard decryption), a Secure Storage Locker (average decryption), a Medical Station (easy decryption), a Malfunctioning Object, and a Wetware kit in the room. Make sure to grab them all before leaving. When you are ready, return to the Normandy to complete the assignment.
If you didn't bring Garrus along, you can have a conversation about the assignment with him after it is completed, although it will be very different in a way that is similar to the conversation with Wrex if you didn't bring him along on his assignment.