He is portrayed by Aidan Quinn.
Martin B is an assassin working for Rodney Cole who steals the identity of Dr. Martin Harris as part of a terrorist plot to murder Professor Leo Bressler, as his death would allow Martin B and the other assassins involved to steal his research and gain a fortune. Throughout the film, Martin tries to convince eberyone that he is the real Martin and that Martin B is an impostor. Martin's wife, Liz, had secretly been involved in the terrorist plot as well and accompanies Martin B to a hotel where Bressler is staying. Liz tries to plant a bomb inside the building but accidentally blows herself up, and Martin manages to prevent Martin B from killing Bressler and the pair of them fight. Martin eventually gains the advantage by smashing a mirror and grabbing one of the broken shards of glass, which he then uses to slash and stab Martin B in the throat, ending his life.