When combined with the dictionary form of verbs, ところだった will indicate that something was about to happen, and will regularly be translated as ‘was just about to do (A)’, or ‘was in the middle of (A)’. In many cases, ところだった will indicate either a positive or negative result that came about from ‘not doing (A)’.
As this grammar pattern is a combination of ところ ‘place’, and the past tense of だ, だった; it will put put emphasis of ‘having been in the place or situation of (A)’, before something happened that caused the expected result to change.

• 危（あぶ）なかった。危（あや）うくあの人（ひと）をひくところだった。
  That was close. I was about to run over that person.
• 起（お）きるのが3分（ぷん）遅（おそ）かったら、遅刻（ちこく）するところだった。
  If I woke up 3 minutes later, I would have been late.

In cases where a positive result was narrowly missed out on, のに will often appear directly after ところだった. This can be interpreted as ‘despite almost doing (A), (B)’.

• 今（いま）から家（いえ）を出（で）るところだったのに、いきなり雨（あめ）が降（ふ）ってきたから雨（あめ）が止（や）むまで待（ま）つ。
  I was just about to leave the house now, but it suddenly started raining, so I am going to wait until the rain stops.
• もうすぐ犯人（はんにん）が自白（じはく）するところだったのに、お前（まえ）が余計（よけい）なことを言（い）うから...。
  The killer was just about to confess, but you went off on a tangent...