"Castle De'ath" is an unusual Avengers entry, in that Steed and Emma are both undercover at the same place at the same time, from the very start of the episode.  The cinematography is curiously rather bleached, for what probably should have been a starker melodrama  -  but it works splendidly, anyway.  This one consists mostly of skulking and good fights, Emma's skulking done in a fetching peignoir by moonlight.  The flirtation between Emma and Steed is especially light and playful.  The plot, about a secret submarine base and its connection to the disruption of the local economy, is clever and credibly handled.