It starts off with a bang, with the giant whale smashing the Bryces' ship like balsa wood (which, come to think of it, it probably was.) Mrs. Bryce does not get nearly enough credit, having somehow kept her badly injured husband afloat and alive until they were rescued.
Nelson, at this stage in the series, is still in major scientist mode, and very excited at the idea of the knowledge that can be gained by assisting the Bryces. Crane is understandably reluctant to use his high-tech sub as a whale-chaser, although, as it turns out, any lesser sub or ship would probably not have survived.
The interactions between the Bryces quickly make it clear that Mr. Bryce has lost all interest in the scientific aspects of his pursuit. He cannot accept the fact that his insistance on pushing forward resulted in his son's death--so he has shifted all blame to the whale. When Mrs. Bryce attempts to force him to see things as they are, he strikes her down in a truly shocking moment of violence.
Nelson's behavior is far less understandable. It's all very well and good to get caught up in the scientific excitement of it all. However, when Mrs. Bryce (who takes an inexplicably long time to try and speak with him) finally reveals her husband's true motivations--Nelson refuses to believe her! He seems to think that he knows Bryce better than Bryce's wife, who has lived and worked with the man at least long enough to produce a teenaged son. When the Seaview has been battered nearly to pieces and is being held together with duct tape and prayer, Nelson (along with Bryce) wants to keep right on with the hunt, nearly resulting in a fight with Captain Crane. It doesn't seem to occur to him that they can always come back after repairs have been completed. They now know the whale's home ground and they can track it with the EKG device.
Crane's forcing Captain Ahab at him gives Nelson pause for thought, but it's not until he finds Bryce preparing to leave the sub that he finally acknowledges that something is seriously wrong. Even then, he clings to Bryce's scientific importance as an excuse to risk 125 lives. (No explanation as to why they couldn't have used the mini-sub or the diving bell, and spared Seaview the dangerous dive.)
Going after that whale with one little 'ol harpoon was rather like trying to lance an elephant with a toothpick, and Bryce comes to an appropriate, Captain Ahabesque end. (Even if he'd managed to get loose, the ensuing bends from being yanked 300 feet to the surface would probably have finished him off, more unpleasantly than drowning.)
Mrs. Bryce, who has probably cried herself out over the past six months, deals with the loss better than Nelson, and the episode ends on a sad, thoughtful note.