Causal
strong strength
Narrative Connection
How these two moments in the story relate
From
Tegan turns against Nyssa in the marketplace
S20E6
·
Snakedance Part 2
To
Doctor confirms Tegan bears Mara mark
S20E6
·
Snakedance Part 2
Why These Connect
The narrative assertion
"Nyssa's initial encounter with Tegan in the marketplace—where Tegan’s erratic behavior is first observed—directly leads to the Doctor's realization of the Mara's influence when Nyssa reports back, prompting him to examine the anti-dream device and notice the mark on Tegan's arm."
About Causal Connections
A directly causes B. The first event sets forces in motion that produce the second. These are the load-bearing connections of plot--remove one and the story structure collapses.