SMT: Devil Summoner is a ‘branch’ of the Megaten (Shin Megami Tensei) ‘family’ so it differs somewhat from them. The most obvious difference being the real-time combat, but I think it suits this particular game.
You play a Radiou Kuzunoha 14th. The latest in a long line of men who are able to trap Demons and then summon them later to do his bidding, whether it's to help in combat, sneak into places Raidou cannot go or helping to make a reluctant person talk. And talk is what you need people to do, because Radou is a detective. The game is laid out in chapters with a crime to solve in each chapter, but each crime is merely a part of something much larger.
As usual Kazuma Kaneko's stunning character design makes SMT: Devil Summoner a gorgeous looking game to play. And it's fast paced and fun as well, with the same Demon Fusion that SMT: Lucifer's Call has.
The only negative with this stunning game is that the combat is real-time instead of the press-turn, turn-based, combat of the ‘regular’ Megatens. But that's just my opinion – you may like real-time combat.