As the story opens Detective Mary Lancester, Decker's old partner, tells him a homeless man named Sebastian Leopold has confessed to murdering Decker's family. Decker, posing as an attorney, manipulates his way into the jail to talk to Leopold. The homeless man, who appears to have mental problems, claims he killed Decker's family because the detective dissed him at a 7-Eleven. Just when Decker is sneaking into the jail to speak to Leopold a mass shooting occurs at local Mansfield High School, killing a number of staff and students.
Because he was an excellent detective Decker is asked to consult with authorities on the Mansfield High School murders. He uses his extraordinary memory to help the cops and FBI search for the high school shooter and to look into Leopold's bogus-sounding confession. Decker uncovers one clue after another in the mass shooting case, which turns out to be an intricate plot that has a connection to the murder of Decker's family. Along the way Decker, a socially inept loner, has some hostile interactions with an FBI agent and a newspaper reporter who want his cooperation. Eventually, though, Decker manages to work with others on the investigations.
This is a well-written, engaging, fast-paced mystery that kept my attention as the actions and motives of the perpetrator were exposed. My biggest problem with the plot is that both Decker - and especially the FBI - seem too slow on the uptake in unraveling some of the clues. Without giving away any spoilers I'll just say that, in real life, the FBI would probably have exposed/understood some clues well before Decker had his 'aha moments'. This may be necessary for the story's plot but it doesn't ring true. Moreover, given the motive, I would have expected the perpetrator to do some things quite differently.
Still, the book kept my interest from beginning to end and I would read more books about detective Decker's exploits. I'd recommend Memory Man to mystery fans.