LLL4 - The Sea Devils 4

Captain Hart and the Navy play the role of substitute Brigadier and UNIT, respectively, very well in this story. In fact, they almost play it too well. By the beginning of Season 9, the relationship between The Doctor and the Brigadier had become so familiar that The Doctor no longer had to try and convince the Brigadier of the dangers of an approaching alien menace. The Brigadier knew enough to just believe his scientific advisor and suitably adjusted his strategy to accommodate. Of course, by now that strategy often extended to simply staying out of the way while The Doctor sorted it out.

With a new authority figure in Captain Hart, The Doctor is forced to start from scratch in trying to convince Hart that the Sea Devils are a threat. While Hart sees enough proof, and the thus act more quickly than the Brigadier might have in a similar situation, Hart's Navy does seem at least more competent in this situation than UNIT ever did. (This may have more to do with the fact that not only were the Navy extras playing soldiers were actual Navy personnel, but also because the production were keen to show the Royal Navy in a good light in exchange for their assistance in the production of the serial).

Season 9 is not the Brigadier's or UNIT's finest hour, and The Sea Devils is the ultimate slap in the face because it shows that their roles can not only be replaced in a story quite easily, but greatly improved upon as well.