OK, time to do this. I haven't been playing Mass Effect 3, so I was putting this off, but meh, might as well give my thoughts on the mission before they go completely poof in my mind.
As mentioned, Shepard goes in with Tali (required) and Garrus (because ride or die, and it just feels right having those three on this mission) to head to the source of the Reaper Code signal and blow it up with an orbital strike. It's the only way to be sure, after all. Of course, they can't just go straight there, that would be easy. They have to drop a bit aways, then go in on foot, while keeping in communication with Legion to get updates on everything.
However, when Shepard and her party get to the energy signature that indicates where the signal is coming from and orders the strike, it turns out that the source of the code isn't merely a computer or something. It's a full blown Reaper. Welp, time to get the fuck out of there...at least for a while. After a bit of running, Shepard decides that it is time to stop running and stand their ground. This ends here! Um, yeah, fighting a Reaper on foot. That will go well. I mean, yes, we have the quarian fleet and the Normandy, but it took throwing literally everything the asari, turians, and the Alliance had to take down Sovereign in Mass Effect, and even then there were some serious casualties. Surely, they can't just have the quarian fleet and Normandy alone take down a Reaper.
So, after Shepard uses the targeting beacon to sync up the quarian fleet and Normandy to fire in unison, there's a boss fight where Shepard has to slooooowly target the Reaper as it charges up and fires its lazers. Needless to say, letting the beam of death that can easily destroy a starship touch Shepard is no bueno. It should be tense, but between how slowly the targeting beacon actually closes in and how clunky dodging to avoid the laser beam feels, it's more annoying. Three volleys later, the Reaper is mortally wounded. I guess the quarian fleet is better than every Council fleet combined. Noted. After a rather standard "You have no idea what you are dealing with, mortal" speech, Reaper goes bleh. So, in the course of one game, we have seen a relatively small fleet take down a Reaper in a few volleys, and another Reaper get taken down by literally the mother of thresher maws. Man, Sovereign must have just been that badass to deal with everything he did in Mass Effect 1 by Reaper standards, because these guys are going down a fair bit easier.
After that, Legion decides to try to upload the code to the other geth now that the Reaper corruption is dealt with, giving full self-sufficiency/independency to the geth. This is not necessarily a problem, as long as the quarians don't attack. OH COME ON HAN'GERREL ARE YOU KIDDING ME? Yep, Mr. Let's Do The Whole Village decides that now is the perfect time to attack. So now, Shepard has a choice: allow Legion to finish the upload, and have the geth destroy the quarians in retaliation, or stop Legion and let the quarians blow the geth away.
Or, if you've made the right choices throughout the trilogy, you manage to have a third option. You see, there is a hidden set of flags that add up to, IIRC, 6 points total. You get some points for not getting Tali exiled, some points for brokering a peace between Tali and Legion during their disagreement in Mass Effect 2, some points if you destroyed the geth in Legion's loyalty mission rather than rewriting them, and some points if you saved Zaal'Koris. Get enough points, and you can allow Legion to upload the code, while convincing the quarians to break off the attack. The tone changes, however, between Paragon and Renegade. Renegade Shepard is basically "I have saved your asses more than enough times. If you fuck this up, I'm NOT saving you this time." That convinces the quarians to hold back until Legion uploads the code. Sadly, to disseminate the code, Legion also has to add his own runtimes to the process. In essence, Legion sacrifices himself. It's a pretty sad moment, and unlike Mordin, there is legitimately no way around it. Either Legion dies this way, or if you stop him from uploading the code, Tali is forced to kill him to save Shepard. This way does have pretty sad last words from Legion as well, when Tali tells Legion the answer to the question "Does this unit have a soul?" was "Yes." "I know, but thank you, Creator-Zorah. Keelah'selai."
After that, the newly independent geth decide to work with the quarians to help them resettle into Rannoch, with Tali eventually deciding to join Shepard until after the Reapers are dealt with. After that, Shepard gets a message from the asari councilor, who wants to share some very private information with Shepard that may help with the war. So, off to the Citadel to meet with the councilor.