N3 — outside your level filter

and then; whereupon — what happened next, unbidden

玄関げんかんのドアを開あけた。すると、知しらない猫ねこが飛とび出だしてきた。

genkan no doa o aketa. suruto, shiranai neko ga tobidashite kita.

I opened the front door. Whereupon a cat I'd never seen came flying out. neutral

すると、明日あしたの集あつまりには来こられないということですね。

suruto, ashita no atsumari ni wa korarenai to iu koto desu ne.

So what you're saying is you can't make tomorrow's gathering. neutral

Variants

そうするとsousurutoalternant
spells out the そう it refers back to; preferred when the previous sentence describes an ACTION the listener might also take (ボタンを押おします。そうすると…), and it carries both the narrative and the working-it-out sense.
そうしたらsoushitaracolloquial
the everyday spoken one, often clipped to そしたら — as a narrative connective it tells what happened next, and it does not carry the working-it-out sense.

すると

するとsurutoN3neutral