At the Bannisters’ house on the night of the fake murder, Broome reveals that he’s rumbled Grisby’s plan, which is in fact to kill Bannister, pin the murder on Michael and run off with Elsa. When he tries to blackmail Grisby, the latter shoots him, then goes out and hands the gun to the unsuspecting Michael to use for the fake murder.
While Grisby and Michael make for the Sausalito dock, Elsa, who has heard the gunshot, finds Broome dying on the kitchen floor and listens unmoved as he tells her about the plot to murder her husband.
At the dock, Grisby smears some of his blood (he’s been cut when the car windscreen was smashed on the way) onto Michael’s clothes and then sails off in a speedboat. Only when he phones the Bannisters’ house and hears Broome’s dying words does Michael realize he’s been taken for a ride:
Get down to the office, Montgomery Street. You was framed. Grisby didn’t want to disappear. He just wanted an alibi – and you’re it. You’re the fall guy. Grisby’s gone down there to kill Bannister now.