すると
and then, at that; so (you mean)
Action。すると、what happened next。
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すると describes what happened right after an action, often as its result or a discovery: ボタンを押した。すると、ドアが開いた, "I pressed the button. At that, the door opened".
The second half is something that happened, usually outside the speaker's control, so it's common in stories and descriptions. It's like the と of "whenever" (N4) turned into a sentence opener.
In conversation, すると also draws an inference from what someone said: 「明日は休みです。」「すると、会議はないんですね。」, "so there's no meeting, then?"
Compare そこで, where the speaker takes action in response. すると is what happened; そこで is what someone decided to do.
Examples
ボタンを押した。すると、ドアが開いた。
I pressed the button. At that, the door opened.
窓を開けた。すると、冷たい風が入ってきた。
I opened the window, and a cold wind blew in.
「明日は休みです。」「すると、会議はないんですね。」
"Tomorrow's a holiday." "So there's no meeting, then?"
薬を飲んだ。すると、すぐに痛みが消えた。
I took the medicine, and the pain went away at once.
猫の名前を呼んだ。すると、猫が走ってきた。
I called the cat's name, and it came running.