In addition to starring, Isaacs will serve as producer on the project, which is described as an Inception-style thriller centered on a cop (Isaacs) who wakes up after an accident to find he is living in two different realities.
The casting of Isaacs lifts the contingency on the pilot, run by former 24 showrunner Howard Gordon.
REM brings the Brit back to 20th TV, where he starred in the drama pilot for Fox Pleading Guilty last season.
Isaacs had been in demand ever since the Showtime drama Brotherhood ended in 2008. He was first pursued in the 2009 pilot season, but was unavailable because of commitments to the Harry Potter movies (in which he plays Lucius Malfoy). Then a year ago, he received eight offers and went with the aforementioned Pleading Guilty.
Isaacs will next be seen in the final Harry Potter film and the Taylor Lautner starrer Abduction.