Semi-Spoilers on the plot of the VM movie
"Murder" isn't a huge spoiler, as it was a recurring theme on Veronica Mars, with the UPN series' first season focusing on the murder of Kristen Bell's character's best friend Lilly Kane (Amanda Seyfried). And from what Entertainment Weekly published in their latest issue, which discusses the Kickstarter campaign for the movie, murder is the planned focus for the film.
(More specific plot spoilers ahead)
EW says the plot will be set a decade after the show's third season and will have Veronica returning to Neptune, California after Logan (Jason Dohring) seeks her help in investigating the murder of his pop-star girlfriend. He's apparently the prime suspect, which doesn't sound all that unfamiliar when we look back at Season 1 of the series.
We also know from what Thomas has said on the Kickstarter page that there may be a 10 year high school reunion at Neptune High, which would certainly provide an excuse to bring some of Veronica's old friends back into the picture. Go pirates!
Also mentioned in the article is Rob Thomas and Kristen Bell's hope to bring some footage of the film to Comic Con this year. If they do manage to get the film started in June, they might have something finished to show Veronica Mars fans at SDCC in July. It would be pretty fantastic if they can make that happen.
EW's full article about the Veronica Mars movie Kickstarter campaign can be read in EW's "News and Notes" column on page 13 of the March 22, 2013 issue (with the Game of Thrones cover).