。
She watched me go into the store.
Literal
She [topic-は] I [subject-が] store [into-に] enter [nominalizer-の] [object-を] saw.
A perception verb (見る 'see / watch') taking a nominalized clause as its object via のを. The pattern [clause]のを見る is the standard Japanese way to express 'watch / see X happening' — Vのを + 見る, 聞く, 感じる, etc. The nominalized form keeps the embedded action ongoing in the perception. Note the が marking the embedded subject 私: inside the relative-like clause, the natural subject marker shifts to が.