ところを/ところに/ところへ

「ところ」expresses the idea of just as something (V) is about to/happening/has just happened, another event interrupts. The particle used with 「ところ」depends on the verb that follows. Specifically:
- 「ところを」: is often followed by verbs meaning "see, discover" such as 見る (to see), 見つける (to find), 見つかる (to be found), 発見する (to discover); or verbs meaning "to stop, catch, attack, help" such as 呼び止める (to call out to stop), 捕まえる (to catch), 捕まる (to be caught), 襲う (to attack), 助ける (to help).
- 「ところに/ところへ」: is often followed by verbs like 行く (to go), 来る (to come), 入れる (to put in), 通りかかる (to pass by), 電話がかかる (to receive a phone call).