Puppet on a String

Hospital Dr. Mellor is attacked by a masked assailant carrying a syringe full of lethal drugs, but manages to survive the attack. He points the finger at rival doctor Thomas Finn, but Finn has an alibi. The next day, Mellor is found dead, but again Finn has a cast-iron alibi. Was Mellor correct in his suspicions? Or is Finn in fact entirely innocent?