Mass Effect 2

Story related. If you want more info, continue reading. Otherwise, know that this will unlock through the natural progression of the story. You can begin Arrival after you have completed the mission on Horizon.

The mission begins with Shepard being contacted by the Alliance and asked to rescue Dr. Amanda Kenson. She has discovered a Reaper artifact, but in the process of researching it was captured by the Batarians. The details of the first two parts of the DLC can be found in Covert Action and Last Stand in that order. This summary will pick up after those two sections are completed.

After awakening, head to the mech control console to take control of a LOKI mech. Kill the technician and then the other LOKI mechs that approach. Cross into the room across the way and blow up the equipment to free Shepard. Collect your armor and weapons before moving on. The next several rooms are all filled with various enemies. You'll have some standard guards mixed in with elite guards and pyros.

You'll eventually reach a communication station where you'll be given a choice to contact the Batarian colonists or the Normandy. The choice doesn't affect the outcome of the DLC (although, it may have an effect in Mass Effect 3). It's just a paragon choice. Whichever you decide to try and contact, your call will be blocked and you will be informed that the station's reactor is about to meltdown.

Make your way down into the station's core. Your goal is to reinsert two coolant rods into the reactor. Again, you'll have to repeatedly fight through the same enemies in every room. Once both rods are back in place and the meltdown is averted, you'll have a final confrontation with the Project's leaders. At the tail end of the confrontation, an explosion will go off and Shepard will be knocked out.

When Shepard regains consciousness, collision with the Alpha Relay will be imminent. After trying to call the Normandy, Shepard discovers that communications are down and your only chance of escape is to reach the communications tower. Upon reaching the exterior of the station, the final group of enemies you'll face are another group of guards and another YMIR mech. After they are defeated, reactivate the communications tower and call the Normandy. After a brief interaction with Harbinger, the Normandy arrives to get you off the asteroid just in time.