1- Wesker works with Umbrella
2- He recieves a proposal from The Agency/ The Organization, in exchange for some data about Umbrella and the B.O.W.s. One of the reasons he sends the STARS into the Arcklay facilities is to test the combat data of the B.O.W.s against trained humans. He pretends his death using the virus Birkin gave him, coming back to life much more powerful. Unfortunately for him, some of the STARS survived.
3- Working for the Agency (Umbrella thinks he's death), Wesker sends Ada to the infested Raccoon city, to get a G virus sample.
4- The Agency attacks Rockfort island, releasing the t-virus in it. Wesker explores the island in search for a t-Veronica virus sample. In his way, he finds Claire and Chris.
5- Meanwhile, Umbrella falls and Wesker recieves a new proposal from the pharmaceutical corporation "S", which is rival to Umbrella and the Agency.
6- Still working for the Agency, intending to betray them, Wesker sends Ada and Krauser to retrieve a master plaga sample. Ada somehow knows about Wesker's deal with "S" and, as told by the Agency, sends him a fake sample.
7- Wesker is working with Tricell (dunno much more about it cause i havn'tplayed RE5 yet)