Among all the captivating characters in Slow Horses, Catherine Standish shines the brightest to me because I see her as the personification of the Slough House.

Standish is loyal, organized, timid, seemingly naive, and mellow. You’d think she'd never do anything out of the ordinary. She’s a silent river that flows slowly until it has to take a sharp turn. Just like the Slough House itself.

While the show starts with Cartwright and his failed task, Saskia Reeves’ Catherine Standish leads us into the Slough House, a god-forsaken place for agents that fucked up.

She’s stuck in the secretary/office manager position. However, she clearly has depth and skills under the surface. Her flashbacks explore her emotional baggage as we see her relish in her own ability to work efficiently.

She takes her job seriously. We immediately see that she’s a capable old hand who basically keeps the Slough House running. She, as the show itself, never ceases to amaze: what else can she do?

In S1, she uses her sweet demeanor and tricks only a proficient secretary would know how to pull off to track criminals. In S2, she overcomes her doubts about herself and beats a high-stakes chess game in exchange for precious intel. In S3, she becomes the central figure in the narrative as she uses her interpersonal skills to charm her kidnappers.

As the series progresses, Standish becomes increasingly sure of herself, shedding that timid veil until she bursts into an atypical (albeit deserved) anger state.

With every season, we uncover plots within plots in MI5. This masterful storytelling technique helps us circle back into unveiling just another bit of Charles Partner’s account, which contains bits of Catherine’s life, too.

While we find her collaborating with almost everyone, we see a special “weird and twisted codependency” with Jackson Lamb, the boss of Slow Horses. Cathrine’s loyalty and dedication to work have made her almost irreplaceable for Lamb and the whole office. Her compassion and empathy almost make her an extension of Lamb in situations where his EQ just won’t cut it.

We also see how essential Standish is to the office's day-to-day. In S3, Lamb looks like he lost a limb when Catherine doesn’t show up at work. Catherine always follows the same routine, so something must be wrong when she fails to do so. He’ll go to hell and back to save Standish.

At the end of S3, Catherine learns that the late Charles Partner, her long-time boss and knight on a white (quick) horse, was a traitor. She doesn’t react well, drops an uncharacteristic F-bomb, and quits the Slough House. What is she going to do in S4?

I’m almost sure that S4 is going to concentrate on how Standish needs the Slough House, just as S3 focused on the opposite.

If we consider her emotional baggage, following exact patterns and being organized might be a coping mechanism. She gives care because she needs care, so her leaving the Slough House is most likely going to throw her off balance and off the wagon.

Catherine is Slough House personified. Her story, with its light and dark sides, lies deep within MI5's history. So, the more we learn about her, the more we know about MI5, and the more we learn about MI5, the more we know about her. Plots within plots within plots, which Apple TV+ just knocked out of The Park. Literally.