Stop the Collectors

The Stop the Collectors mission is the main plot mission in Mass Effect 2, which mainly consists in answering requests from the Illusive Man that will lead Commander Shepard's team (which will build-up along the way) throughout the Terminus Systems, in a effort to deal, by any possible means, with the uncovered collectors threat.

Walkthrough
The Stop the Collectors mission starts right after Shepard has found evidence that the collectors are the force behind all the recent abductions of entire human colonies in the Terminus Systems and only ends when Shepard attempts to deal, whatever the exact outcome may be, with their base on the other side of the Omega-4 Relay. It is mainly an outline mission which allows to track the advancement of the plot.

Assemble a team (1)
Upon coming back from a field investigation at Freedom's Progress, with Miranda and Jacob, Shepard has a meeting with the Illusive Man, during which Shepard learns that their findings of evidence incriminating the collectors as responsible for the mass abductions of human colonies confirms the Illusive Man's pre-existing suspicions. Then Shepard, though being more or less enthusiast about working with Cerberus, has to agree that their effort to oppose this new threat upon the Galaxy appears genuine and is worth of Shepard's involvement. The Illusive Man then tell Shepard he should start by assembling a strong team for their now common effort. He explains that most of Shepard's former team has scattered throughout the galaxy and is either unavailable, unaccounted for or difficult to convince to work with Cerberus. He then presents Shepard with a list of possible recruits to consider, based upon Cerberus intelligence (Dossier: Archangel, Dossier: The Convict, Dossier: The Professor, Dossier: The Warlord and, if you have registered the DLC "Cerberus Network", Dossier: The Veteran). They also discuss the question of involving the Galactic Council, which is left to Shepard's appreciation. The Illusive Man then specifies that Miranda and Jacob will continue to serve under Shepard's command. Then, upon ending the meeting, the Illusive Man pleasantly surprises Shepard by providing a first crew member Shepard already knows very well: Joker, who leads Shepard to another very good surprise, their ship: the new SSV Normandy SR-2. This part of the mission ends when you have recruited what is considered a sufficient core team (likely having completed at least 3 of the Dossier missions).

Go to Horizon
This new part starts right after you first task is considered sufficiently completed. An urgent message from the Illusive Man explains that Cerberus has received a warning of a possible new collector attack on the Horizon human colony. He requests that you go there immediately, to attempt to stop the attack, but also, and more importantly, to gain a better understanding of the attacks and to research a way to oppose new ones.

Assemble a team (2)
This part of the main mission requires you to recruit additional squad members, and the following dossiers are added : Dossier: The Assassin, Dossier: Tali and Dossier: The Justicar.

Investigate Collector Ship
You're sent to investigate a disabled collector ship in order to determine how to use the Omega 4 Relay without being destroyed.

You leave after EDI hacks into the mainframe of the ship and discovers they use an IFF to get past the Omega 4 Relay.

Acquire Reaper IFF
You travel to a dead reaper to get it's IFF device. When you arrive you discover it isn't dead only mostly disabled, and as you fight your way to the core and the IFF you get help from an unexpected source.

You obtain the IFF, kill the reaper, and have the option to recruit a new member to your party.

Await IFF Installation
NOTE: If Legion's loyalty mission is acquired prior to accessing the Galaxy map/the IFF install, the "time limit" to the endgame will not begin. If however, you do not acquire Legion's loyalty mission prior to accessing the Galaxy Map. The install will be completed and the "time limit" will begin.

Use Omega-4 Relay
This is the point of no return. Before you use the Omega-4 Relay, make sure that every team member is loyal and the Normandy's fully upgraded with shielding, armor and a bigger weapon. If your ship is not fully upgraded or some of your team members aren't loyal, parts of your crew might die during the suicide mission.

Defeat the Collectors
You've used the relay and landed on the collector base's hull, now you have to go in and destroy it. This mission varies by who is in your party and if you gained their loyalty by doing their side missions. At the end of the mission you finally find out why the collectors were abducting humans.

You kill the collectors and talk with the Illusive Man one last time, this conversation varies based on your final decision (the sequel's version of whether to save the council or not).