For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
The trial of local fisherman Kabuo Miyamoto opens in the winter of 1954. Miyamoto is accused of having murdered another local fisherman, Carl Heine, in the fall. We learn a lot about the characters and the events relevant to the trial through flashbacks. We also get the back story of a local reporter, Ishmael Chambers, and details about his relationship with the defendant's wife, Hatsue, when they were teenagers.
Act II
The trial moves into Day 2 with more testimony for the prosecution. The prosecution rests at the end of that day, right after a snowstorm knocks out the power on San Piedro (the island where the story is set). After court recesses for the day, Ishmael heads out to do some research at a local coast guard office for a story on big storms (and to visit his mama, too).
While doing his storm research, he realizes he should dig around for any information that could be relevant to the Miyamoto trial (since the alleged murder took place out on the water). Within just a few minutes, he finds information that could potentially exonerate Miyamoto. He's not compelled to do anything with it, though, probably because he's still pretty interested (romantically) in Kabuo's wife.
Act III
We enter Day 3 of the trial, when the defense makes its case. Enough evidence comes to light to raise reasonable doubt, but when the jury finally enters deliberations, they are almost unanimous in wanting to convict.
Meanwhile, Ishmael finally gets it together and realizes he needs to come forward with that evidence he found in the coast guard office. That touches off a chain of events that end with Kabuo being exonerated and returned to his family. Ishmael, meanwhile, returns to his office to write up the story of the whole thing, having (finally) given up on pursing Hatsue.