A mysterious man who assumes different identities uses them to infiltrate crime scenes and try to solve them within the first 72 hours so that he can go back in time 3 days and both stop them from happening and catch the perpetrators.  But it turns out that he is only able to do this because of a device that he stole from a consortium of very powerful people (who want the device back for their own hidden purposes), that he must continually elude if he is to continue helping people.