Season 1 of The Wire is the Barksdale detail: Avon is headed to prison and McNulty is punished to the boat.
Stringer Bell is still alive and free, the street brain of the organization Avon can no longer walk.
Season 2 is the port: the Sobotka union is broken. Avon is still inside. Stringer still runs the free-world Barksdale shop.
The Wire's third season is about reform experiments and their failure modes: free zones, chain of command, and the cost of 'clean' corners on paper.
Stringer's attempt to corporatize the drug trade collides with Avon and street rules. Omar and Mouzone close a ledger.
Barksdale detail; Avon and Stringer.
D'Angelo's tragedy; Wallace killed.
Major case partial wins.
Stevedores and the Greek.
Frank Sobotka destroyed.
Port economy rot.
Hamsterdam rises and falls.
Stringer killed.
Carcetti politics rise; Marlo ascends.
Season bridge: everything above is end of Season 3. S04E01 ("Boys of Summer") has not started.
Cutoff discipline: every fact above ends at S03E12. Do not import anything from S04E01 ("Boys of Summer") or later.