I/you/we/she/it/ we/they had= I'd etc./ he'd + past participle (gone/seen/ finished)
If we want to talk about things that happened before an action in the past, we use the past perfect.
e.g. When I went to Sue's she had gone out.
The man sitting next to me on the plane was very nervous. He'd never flown before.
We arrived to work in the morning and found that somebody had broken into the office during the night.